An introduction to Perl Best Practices, from YAPC::Asia 2009

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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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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126. Perl Best Practices
@M" I.'(9)*'$.&8$'.(,8(/=+$8C'(,-'($'%*%?/5/,4(8<(58)>()9=?'$."
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Documentation
D!" I.'(%(-'$'*8&(;-')(%(=95,/5/)'(.,$/)>('G&''*.(,;8(5/)'." LF" A/.,/)>9/.-(9.'$(*8&9='),%,/8)(<$8=(,'&-)/&%5(*8&9='),%,/8)"
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,/8)"
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!OJ" I.'(DN%$>(#-:#*%(/).,'%*(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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!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