There are more efficient ways to implement this algorithm. The implementation you've presented is n squared. That is, as input.txt or library.txt become longer, the program takes exponentially more time to complete. If the files are small, this won't be a big deal. If the files are large, you can make this run much more efficiently by using hashes.

Hashes are the fastest way to compare the contents of one list against another list. It is an n algorithm, which means you only take a linear hit in run time as input.txt or library.txt grow.
$library = [contents of library.txt]
$input = [contents of input.txt]

my %in_library = ();
foreach $entry (split(/\n/, $library)) {
  $in_library{"$entry"} = 1;
}

my @input = split(/\n/, $input);

foreach $entry (@input) {
  if ($in_library{"$entry"}) {
    print "found - $entry\n";
  } else {
    print "new - $entry\n";
  }
}

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Regular expressions take time to compile, so you want to avoid putting dynamic regexps in a tight loop.

TROGDOR

In reply to Re: Pattern matching with a variable by TROGDOR
in thread Pattern matching with a variable by Anonymous Monk

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