This document introduces best practices for writing clean and readable Perl code. It provides examples of poorly formatted code and discusses improvements like using strict and warnings, consistent indentation, descriptive variable names, and limiting line length to 80 characters. The examples demonstrate separating code into logical blocks, spacing around operators, and vertical alignment to improve readability. Adopting these styles and standards helps code be more maintainable as projects evolve over time.

1. Introduction to
Perl Best Practices
José Castro <cog@cpan.org>
Tokyo, September 2009

2. Who am I?
• Perl Hacker from Portugal
• COG@CPAN
• YAPC::EU::20{04,05,06,07,08,09},
YAPC::NA::20{05,06,07,08},
YAPC::Asia 20{08,09}, LPW 20{04,05},
PPW 2008, OSDC::AU 2006
• Technology Evangelist @ SAPO
SAPO/Portugal
Telecom

3. What is SAPO?

4. What is SAPO?

5. What is SAPO?
• Older than Google,
kind of like Yahoo!

6. What is SAPO?
• Older than Google,
kind of like Yahoo!
• 100+ techs

7. What is SAPO?
• Older than Google,
kind of like Yahoo!
• 100+ techs
• 100+ non-techs

8. What is SAPO?
• Older than Google,
kind of like Yahoo!
• 100+ techs
• 100+ non-techs
• Mail, Blogs, Photos,Videos, Links, Cars, Real
Estate, Jobs, Maps, Encyclopedia online,
Search, Messaging, Social Network, etc., etc.,
etc.

9. What do I do at SAPO?

10. What do I do at SAPO?
• Evangelism (motivation, productivity, quality)

11. What do I do at SAPO?
• Evangelism (motivation, productivity, quality)
• Organize the training program

12. What do I do at SAPO?
• Evangelism (motivation, productivity, quality)
• Organize the training program
• Recruitment (lots of interviews every week)

13. What do I do at SAPO?
• Evangelism (motivation, productivity, quality)
• Organize the training program
• Recruitment (lots of interviews every week)
• Organize events

14. What do I do at SAPO?
• Evangelism (motivation, productivity, quality)
• Organize the training program
• Recruitment (lots of interviews every week)
• Organize events
• Etc.

15. Introduction to
Perl Best Practices
José Castro <cog@cpan.org>
Tokyo, September 2009

16. “What?”
• a set of suggestions/guidelines to improve
your code’s readability and reliability

17. Disclaimer
• what works for a particular situation may
not be adequate for another
• YMMV

18. “Why?”
• tomorrow, you may need to fix a glitch in
your code
• next week, you may need to confirm
something
• 6 months from now, you may need to
develop a new feature

19. “How?”
• using standards
• defining your best practices and following
them

20. Code Sample #1

21. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

22. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

23. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

24. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

25. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

26. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

27. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

28. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

29. Block separation
• A program is a list of instructions
• You’re writing it as a letter
• Separate your paragraphs, for clarity

30. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

31. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

32. Separate instructions
• Each instruction should have its own line

33. use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

34. use strict;
my @list=('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

35. Indentation
• 2 spaces?
• 4 spaces?
• 6, 8?
• Tabs?

36. use strict;
my @list=('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

37. use strict;
my @list=('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

38. Spaces around
operators
• Don’t clutter things up
• Spaces around operators help you find
them more easily

39. use strict;
my @list=('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

40. use strict;
my @list=('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

41. use strict;
my @list = ('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i = int ($rand + rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

42. Naming variables
• Don’t name them “var”, “list”, “hash” or
anything else that does not have a meaning
• Make sure the name helps you identify
what it is

43. use strict;
my @list = ('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i = int ($rand + rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

44. use strict;
my @editors = ('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i = int ($rand + rand());
print " $editors[$i] users are from Venus" . ",
"$editors[1-$i] users are from Mars" .
"n";
}

45. Vertical indentation
• Vertical indentation helps your eyes find
things more easily

46. use strict;
my @editors = ('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i = int ($rand + rand());
print " $editors[$i] users are from Venus" . ",
"$editors[1-$i] users are from Mars" .
"n";
}

47. use strict;
my @editors = ('Emacs', 'Vi ');
my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i = int ($rand + rand());
print " $editors[$i] users are from Venus" . ",
"$editors[1-$i] users are from Mars" .
"n";
}

48. Code Sample #2

49. my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

50. my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

51. my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

52. my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

53. my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

54. 2 is both a prime number and a fibonacci number
3 is both a prime number and a fibonacci number
5 is both a prime number and a fibonacci number
13 is both a prime number and a fibonacci number
89 is both a prime number and a fibonacci number

55. Strict and warnings
• both strict and warnings will help you avoid
lots of pesky mistakes

56. my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

57. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

58. Avoid lines over 80 charac
• code is easier to read if it fits on the screen
• even if using line wrapping, having to figure
out if two lines are different instructions or
the same one is harder than not having to
do it

59. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,144,233,377,
for my $number (@list) {
if (grep{$number==$_}@list2) {print "$number is
}

60. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,
37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ;
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,
144,233,377,610,987,1597,2584,4181,6765) ;
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number and a
}

61. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,
37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ;
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,
144,233,377,610,987,1597,2584,4181,6765) ;
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " . " a
}

62. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,
37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ;
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,
144,233,377,610,987,1597,2584,4181,6765) ;
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

63. When breaking lines,
align vertically
• a broken line is a visual accident waiting to
happen

64. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,
37,41,43,47,53,59,61,67,71,73,79,83,89,97,101) ;
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,
144,233,377,610,987,1597,2584,4181,6765) ;
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

65. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,
37,41,43,47,53,59,61,67,71,73,79,83,89,9
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,
144,233,377,610,987,1597,2584,4181,6765
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

66. Add spaces after a
comma
• spaces after a comma help you visually
separate items

67. use strict;
use warnings;
my @list = (2,3,5,7,11,13,17,19,23,29,31,
37,41,43,47,53,59,61,67,71,73,79,83,89,9
my @list2 = (1,1,2,3,5,8,13,21,34,55,89,
144,233,377,610,987,1597,2584,4181,6765
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

68. use strict;
use warnings;
my @list = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31,
37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
my @list2 = (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89,
144, 233, 377, 610, 987, 1597, 2584, 41
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

69. Organize long lists
as tables
• the brain identifies patterns such as tables
much more easily

70. use strict;
use warnings;
my @list = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31,
37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
my @list2 = (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89,
144, 233, 377, 610, 987, 1597, 2584, 41
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

71. use strict;
use warnings;
my @list = ( 2, 3, 5, 7, 11, 13, 17, 19, 23, 29,
37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
my @list2 = (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89,
144, 233, 377, 610, 987, 1597, 2584, 41
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

72. use strict;
use warnings;
my @list = ( 2, 3, 5, 7, 11, 13, 17, 19, 23, 29,
37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
my @list2 = ( 1, 1, 2, 3, 5, 8, 13, 2
144, 233, 377, 610, 987, 1597, 2584, 41
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

73. Again, name variables
appropriately
• the brain identifies patterns such as tables
much more easily

74. use strict;
use warnings;
my @list = ( 2, 3, 5, 7, 11, 13, 17, 19, 23, 29,
37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
my @list2 = ( 1, 1, 2, 3, 5, 8, 13, 2
144, 233, 377, 610, 987, 1597, 2584, 41
for my $number (@list) {
if (grep{$number==$_}@list2) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

75. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if (grep{$number==$_}@fibonnaci) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

76. Spaces for clarity
• around operators
• around blocks
• around anything else that may cause
confusion

77. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if (grep{$number==$_}@fibonnaci) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

78. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

79. Block closing
• the closing curly brace should be in a line
by itself
• this helps better identify where the block
ends

80. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern"}
}

81. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern"
}
}

82. Always end a statement
with a semicolon
• even if it’s the last statement in the block
• even if the closing curly brace is on the
same line
• this will allow you to add another
statement without having to check the line
above

83. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern"
}
}

84. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}

85. Inside a loop, use empty
lines around the body
• this is also going to help you identify the
beginning and the end of the loop

86. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}

87. use strict;
use warnings;
my @prime_numbers = ( 2, 3, 5, 7, 11, 13, 17, 19,
37, 41, 43, 47, 53, 59, 61, 67,
my @fibonnaci = ( 1, 1, 2, 3, 5, 8,
144, 233, 377, 610, 987, 1597,
for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}

88. Code Sample #3

89. for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}

90. if ( grep { $number == $_ } @fibonnaci ) {

91. Using CPAN
• Many modules on CPAN already implement
what we need in a far better way than we
could possibly do in a couple of minutes

92. grep { 2 == $_ }
( 1, 1, 2, 3, 5, 8, 13,
21, 34, 55, 89, 144, 233, 377,
610, 987, 1597, 2584, 4181, 6765, 10946)
if ( grep { $number == $_ } @fibonnaci ) {

93. grep { 2 == $_ }
( 1, 1, 2, 3, 5, 8, 13,
21, 34, 55, 89, 144, 233, 377,
610, 987, 1597, 2584, 4181, 6765, 10946)
if ( grep { $number == $_ } @fibonnaci ) {

94. grep { 2 == $_ }
( 1, 1, 2, 3, 5, 8, 13,
21, 34, 55, 89, 144, 233, 377,
610, 987, 1597, 2584, 4181, 6765, 10946)
if ( grep { $number == $_ } @fibonnaci ) {

95. grep { 2 == $_ }
( 1, 1, 2, 3, 5, 8, 13,
21, 34, 55, 89, 144, 233, 377,
610, 987, 1597, 2584, 4181, 6765, 10946)
if ( grep { $number == $_ } @fibonnaci ) {

96. grep { 2 == $_ }
( 1, 1, 2, 3, 5, 8, 13,
21, 34, 55, 89, 144, 233, 377,
610, 987, 1597, 2584, 4181, 6765, 10946)
if ( grep { $number == $_ } @fibonnaci ) {

97. grep { 2 == $_ }
( 1, 1, 2, 3, 5, 8, 13,
21, 34, 55, 89, 144, 233, 377,
610, 987, 1597, 2584, 4181, 6765, 10946)
if ( grep { $number == $_ } @fibonnaci ) {

98. grep { 2 == $_ }
( 1, 1, 2, 3, 5, 8, 13,
21, 34, 55, 89, 144, 233, 377,
610, 987, 1597, 2584, 4181, 6765, 10946)
n iterations, always
if ( grep { $number == $_ } @fibonnaci ) {

99. if ( grep { $number == $_ } @fibonnaci ) {

100. use List::MoreUtils qw(any);
if ( grep { $number == $_ } @fibonnaci ) {

101. use List::MoreUtils qw(any);
if ( any { $number == $_ } @fibonnaci ) {

102. Structure
• separate your code in subroutines
• subroutines should have meaningful names

103. use List::MoreUtils qw(any);
if ( any { $number == $_ } @fibonnaci ) {

104. use List::MoreUtils qw(any);
if ( is_fibonnaci($number) ) {
sub is_fibonnaci {
my $number = shift;
return any { $number == $_ } @fibonnaci
}

105. for my $number (@prime_numbers) {
if ( grep { $number == $_ } @fibonnaci ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}

106. for my $number (@prime_numbers) {
if ( is_fibonnaci($number) ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}

107. for my $number (@prime_numbers) {
if ( is_fibonnaci($number) ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}
sub is_fibonnaci {
my $number = shift;
return any { $number == $_ } @fibonnaci;

108. for my $number (@prime_numbers) {
if ( is_fibonnaci($number) ) {
print "$number is both a prime number " .
" and a fibonacci numbern";
}
}

109. for (@prime_numbers) {
if ( is_fibonnaci($_) ) {
print "$_ is both a prime number " .
" and a fibonacci numbern";
}
}

110. Code Sample #4

111. for ( 1 .. 1000000000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

112. for ( 1 .. 1000000000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

113. for ( 1 .. 1000000000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

114. for ( 1 .. 1000000000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

115. for ( 1 .. 1000000000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

116. for ( 1 .. 1000000000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

117. Underscores
for large numbers
• Large numbers are hard to read
• Underscores every three digits can help

118. for ( 1 .. 1000000000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

119. for ( 1 .. 1_000_000_000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

120. Booleans
• flags shouldn’t be named “flag”
• they should have intuitive names

121. for ( 1 .. 1_000_000_000 ) {
my $flag = 1;
$flag = 0 if is_prime( $_ );
$flag = 0 if is_fibonnaci( $_ );
if ( $flag ) {
print "$_ seems interestingn";
}
}

122. for ( 1 .. 1_000_000_000 ) {
my $is_interesting = 1;
$is_interesting = 0 if is_prime( $_ );
$is_interesting = 0 if is_fibonnaci( $_ );
if ( $is_interesting ) {
print "$_ seems interestingn";
}
}

123. Tools and helpers

125. Perl Best Practices
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DO" X%4(89,(=95,/5/)'(.,$/)>.(8C'$(=95,/+5'(5/)'."
Documentation
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Reference Guide D@"
,/8)"
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8+'$%)*." F6" A8)B,(9.'(+%&:%>'(C%$/%?5'.(/)(489$(8;)(*'C'58+='),"
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FD" P)/,/%5/0'(%)4(C%$/%?5'(489("('+"/0'"
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66" V)<8$&'(489$(&-8.')(5%489,(.,45'(='&-%)/&%554"
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HJ" TC8/*(SN.,45'(&(%(.,%,'='),." !!@" S%55(.9?$89,/)'.(;/,-(+%$'),-'.'.(?9,(;/,-89,(%(5'%*/)>(9"
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6@" I.'(>$%==%,/&%5(,'=+5%,'.(;-')(<8$=/)>(/*'),/</'$." HM" R'C'$(.9?.&$/+,(=8$'(,-%)(8)&'(/)(%(588+" !!F" T5;%4.(9)+%&:(:)(</$.,"
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6L" A/.,/)>9/.-(*/<<'$'),(+$8>$%=(&8=+8)'),.(?4(&%.'" JD" I.'(&(%(/).,'%*(8<(4+5(;-')(,$%).<8$=/)>(%(5/.,(/)(+5%&'" !!M" T5;%4.($',9$)(.&%5%$(/)(.&%5%$($',9$)."
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@L" A8)B,(+%*(*'&/=%5()9=?'$.(;/,-(5'%*/)>(0'$8."

126. Perl Best Practices
@M" I.'(9)*'$.&8$'.(,8(/=+$8C'(,-'($'%*%?/5/,4(8<(58)>()9=?'$."
DO" X%4(89,(=95,/5/)'(.,$/)>.(8C'$(=95,/+5'(5/)'."
Documentation
D!" I.'(%(-'$'*8&(;-')(%(=95,/5/)'(.,$/)>('G&''*.(,;8(5/)'." LF" A/.,/)>9/.-(9.'$(*8&9='),%,/8)(<$8=(,'&-)/&%5(*8&9='),%,/8)"
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!OJ" I.'(DN%$>(#-:#*%(/).,'%*(8<(5C%59'(#-:#*%" Modules
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Control Structures
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HH" A8)B,(9.'(-)"!##(8$(-)*1"(%,(%55" Subroutines
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!!J" I.'(6!&1)!6)'..(8$(!71#*')&'(,8(,'.,(<8$(=/../)>(%$>9='),." Testing and Debugging
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LO" Expressions !6@" T5;%4.($',9$)(C/%(%)('G+5/&/,(%!*-%)" ">M$ L1;3C&(*0,5(.5(;3,5)54&('AB1)<)*1C$
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!M7$ E,'9'5*(0&'1'&&(D3<=*,3<=)54$
'5<'$ !"#$%&"'(%!#)*(+*"'%,"-"#".*"%/0+1"%2"#'+3.%45645665
!N?$ E,'6',(6)A'/2&*,)54(#1(<.-B3,)&.5&(*.(6)A'/2B3**',5(,'4'A(-3*<+'&$ "!!$ O)&*)540)&+(3,40-'5*&(6.,(D3&'(<13&&'&(DC(<13&&(53-'(3&(;'11$ !"#$%&"'(%!#)*(+*"'%78%,)-+).%/0.1)2
"!"$ I'B3,3*'(C.0,(<.5&*,0<*).5:()5)*)31)V3*).5:(35/(/'&*,0<*).5(B,.<'&&'&$ 9%:66;%<=,"+$$8%>"1+)%?.*5@%A$$%#+BC('%#"'"#2"15
Error Handling "!>$ T0)1/(*+'(&*35/3,/(<13&&()56,3&*,0<*0,'(30*.-3*)<311C$ 3"4"#".*"%56+7"%78%809).%:#0-).'
!N!$ U+,.;('A<'B*).5&()5&*'3/(.6(,'*0,5)54(&B'<)31(9310'&(.,(&'**)54(6134&$ "!J$ %&'(;&:<<##/7I(*.(30*.-3*'(*+'(/'311.<3*).5(.6(3**,)D0*'(/3*3$ 9%:66D%EF0+##"$%G3.'0$().*8@%A$$%#+BC('%#"'"#2"15

127. • Perl::Tidy - “Parses and beautifies perl
source”
• Perl::Critic - “Critique Perl source code for
best-practices”

128. Perl::Tidy
use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

129. Perl::Tidy
use strict;
my @list = ( 'Emacs', 'Vi ' );
my $rand = rand();
print "10 random users said:n";
foreach ( 0 .. 10 ) {
my $i = int( $rand + rand() );
print " $list[$i] users are from Venus" . ", "
. "$list[1-$i] users are from Mars" . "n";
}

130. Perl::Tidy
• configurable (parameters or .perltidyrc)
• integrates with most text editors
• some people use it as an svn hook

131. Perl::Critic
use strict;
my @list=('Emacs', 'Vi '); my $rand = rand();
print "10 random users said:n";
foreach(0..10) {
my $i=int ($rand+rand());
print " $list[$i] users are from Venus" . ", " .
"$list[1-$i] users are from Mars" .
"n";
}

132. Perl::Critic
Loop iterator is not lexical at line 5, column 1.
See page 108 of PBP. (Severity: 5)

133. Perl::Critic
• configurable (several severity levels,
extendable, .perlcriticrc)
• integrates with most text editors
• some people also use it as an svn hook
• if you don’t own PBP, there’s also user-
contributed documentation on each subject

134. Methods
• Pair programming
• Code review

135. Further documentation
• http://127h.sl.pt - An Introduction to Perl
Critic, by Josh McAdams
• http://127i.sl.pt - Perl Tidy Perl Critic, by
olegmmiller
• http://127w.sl.pt - Perl Best Practices
Reference Guide