#!/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>;