my is used to declare and assign the variable $line. If you would 'use strict;' this is a requirement for the script to run.

Manual for my: http://www.perl.com/doc/manual/html/pod/perlfunc/my.html

Edit: Excuse me for this post, I somehow missed that the question was already answered.

In reply to Re^4: how to extract certain lines by Kalaspuff
in thread how to extract certain lines by dee00zee

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