For the record, this is a FAQ.

$ perldoc -q 'change one line in a file'
Found in /usr/lib/perl5/5.6.1/pod/perlfaq5.pod
       How do I change one line in a file/delete a line in a
       file/insert a line in the middle of a file/append to the
       beginning of a file?
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In addition to classics like Learning Perl and The Perl Cookbook, I also like to recommend sitting down one evening and reading through all of the perlfaqs. Even seasoned Perl coders tend to learn new things.