cmpcol -d '|' -us X:3 Y:3 > xy-keys.union

##</code><code>##

1st_key_value <1
2nd_key_value <12
3rd_key_value <2
...

##</code><code>##

#!/usr/bin/perl
use strict;

my %key;
open( C, '<', 'xy-keys.union' ) or die "xy-keys.union: $!\n";
while (<C>) {
    chomp;
    my ( $k, $v ) = ( /^ (.*) < \+? ([12]+) \+? $/x ); # ignore dup.key (+) marks
    $key{$k} = $v;
}

my %common;
open( I, '<', 'X' ) or die "X: $!\n";
open( O, '>', 'X.out' ) or die "X.out: $!\n";
while (<I>) {
    my $k = ( split /\|/ )[2];
    if ( $key{$k} eq '1' ) {
        s/$/|I/;
    }
    else {
        $common{$k} = $_;
    }
    print O;
}
open( I, '<', 'Y' ) or die "Y: $!\n";
open( O, '>', 'O.out' ) or die "O.out: $!\n";
while (<I>) {
    my $k = (split /\|/)[2];
    if ( $key{$k} eq '2' ) {
        s/$/|D/;
    }
    elsif ( $_ ne $common{$k} ) {
        s/$/|U/;
    }
    print O;
}