It's a "diff" problem :)

#!/usr/bin/perl

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

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

my $FileA = <<END;
111 abc 
111 def 
222 ghi 
223 jkl 
345 mno 
END

my $FileB = <<END;
111 pqr 
111 stu 
111 vwx 
222 yza 
345 bcd 
END

my @bigA = $FileA =~ /.*\n/g;
my @bigB = $FileB =~ /.*\n/g;

traverse_sequences( [ map /(...)/, @bigA ], [ map /(...)/, @bigB ],
  {
  DISCARD_A => sub { print $bigA[shift] },
  DISCARD_B => sub { print $bigB[pop] },
  } );
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