21. It's very common to see command line programs that use Unix I/O redirection like this: $> perl -pe 's/PHP/Perl/g' in.txt > out.txt
22.
23. -i[extension] : edit in place. Case extension exists the original file is renamed, otherwise the file is overwitten.
24. If the extension doesn't contain a * , then it is appended to the end of the current filename. If the extension does contain one or more * characters, then each * is replaced with the current filename.
25.
26. It assigns $ to have the value of octnum . Without octnum, $ will be assigned to $/ .
27.
28. Adds $ (the output record separator) to each print