two words: regular expressions.
two more words: not necessarily.

open FILE, "file.txt" or die "can't do : $!\n";
while (<FILE>)
{
   # do some test on $_
   # if it's a string you want, keep it
}
close FILE;
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those tests are most likely going to be some sort of regular expression. but it depends on what you are looking for. what lines do you want to keep? how do you know you want to keep them?