This should get you close, though it assumes the two input files have the same number of lines.

use strict;
use warnings;

my @f1 = get_lines("1.txt");
my @f2 = get_lines("2.txt");

open(my $out1, ">", "3.txt") or die;
open(my $out2, ">", "4.txt") or die;

for my $i (0..$#f1){
   if (abs($f1[$i]->[1]-$f2[$i]->[1])>1){
      print $out1  "$f1[$i]->[0]\n";
      print $out2  "$f2[$i]->[0]\n";
   }
}
close $out1;
close $out2;


sub get_lines {
   my $file = shift;
   my @lines;
   open(my $fh, "<", $file)   or die("Unable to open $file: $!");
   while(<$fh>){
      chomp;
      /(\d+)$/;
      push @lines, [ $_, $1 ];
   }
   close $fh;
   return @lines;
}
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