#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper qw/Dumper/;

my @pairs_in_db = ([1,2],[1,9],[3,4],[5,6]);
my @pairs_from_somewhere_else = ([1,2],[5,6],[7,18]);

my %map = ();
foreach my $pair ( @pairs_in_db ) {
	$map{$pair->[0]}->{$pair->[1]} = 1;
}

my %keep = ();
foreach my $pair ( @pairs_from_somewhere_else ) {
	if( exists $map{$pair->[0]} && exists $map{$pair->[0]}->{$pair->[1]} ) {
		printf("<%s, %s> was in db\n", @{$pair});
		$keep{$pair->[0]}->{$pair->[1]} = 1;
	}else{
		printf("<%s, %s> has to be inserted into the databse\n", @{$pair});
	}
}

foreach my $pair ( @pairs_in_db ) {
	unless( exists $keep{$pair->[0]} && exists $keep{$pair->[0]}->{$pair->[1]} ) {
		printf("<%s, %s> will be removed\n", @{$pair});
	}
}