#!/usr/bin/perl

# http://perlmonks.org/?node_id=1136858

use Algorithm::Diff qw(traverse_sequences);
use strict;
use warnings;

my @bigA = split /\n/, <<END;
111 abc
111 def
222 ghi
223 jkl
345 mno
END

my @bigB = split /\n/, <<END;
111 pqr
111 stu
111 vwx
222 yza
345 bcd
END

my @newfile;

traverse_sequences( [ map /(...)/, @bigA ], [ map /(...)/, @bigB ],
  {
  MATCH => sub { print "$bigA[shift]   $bigB[pop]\n" },
  DISCARD_A => sub
    {
    push @newfile, my $left = $bigA[shift] . "\n";
    print $left;
    },
  DISCARD_B => sub
    {
    push @newfile, my $right = $bigB[pop] . "\n";
    print ' ' x 10, $right;
    },
  } );

print "\n\n", @newfile;
#use File::Slurp; write_file 'newfilename', @newfile; # to make newfile