Anonymous Monk,
Sometimes a problem just needs to be solved quickly and all that matters is getting the result (throw away code). Here is some code that might help get a feel for how quickly you can solve problems with Perl.

#!/usr/bin/perl
use strict;
use warnings;

open (FILE1, '<', 'file1.txt') or die $!;
open (FILE2, '<', 'file2.txt') or die $!;
open (FILE3, '>', 'file3.txt') or die $!;

my ($X) = <FILE1> =~ /^(\d+)/;
print FILE3 $X - $_, "\n" for map { /^(\d+)/ } <FILE2>;
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Of course, memory constraints may mean that this isn't feasible and more thought has to go into it and might end up not being throw-away.