use strict;
use warnings;

my %hash1 = ( ABC123 => '', ABC456 => '' );
my %hash2 = ( ABC456 => '');

my ( @out1, @out2 );

while ( my $entry = <DATA> ) {
  chomp $entry;
  if ( exists $hash1{$entry} ) {
    push @out1, $entry;
  }
  if ( exists $hash2{$entry} ) {
    push @out2, $entry;
    next;
  }
  push @out1, $entry;
}

print "\nOUTPUT1:\n";
print "$_\n" for @out1;

print "\nOUTPUT2:\n";
print "$_\n" for @out2;


__DATA__
ABC123
ABC456
XYZ123
XYZ456

##</code><code>##

use strict;
use warnings;

my %hash1 = ( ABC123 => '', ABC456 => '' );
my %hash2 = ( ABC456 => '');

my ( @out1, @out2 );

while ( my $entry = <DATA> ) {
  chomp $entry;
  my $seen;
  if ( exists $hash1{$entry} ) {
    push @out1, $entry;
    $seen = 1;
  }
  if ( exists $hash2{$entry} ) {
    push @out2, $entry;
    next;
  }
  push @out1, $entry unless $seen;
}

print "\nOUTPUT1:\n";
print "$_\n" for @out1;

print "\nOUTPUT2:\n";
print "$_\n" for @out2;


__DATA__
ABC123
ABC456
XYZ123
XYZ456