#!/usr/bin/perl
use strict;
use warnings;
use Graph;

my $g = Graph::Undirected->new;

while (<DATA>) {
	chomp;
	my ($x, $y) = split /;/;
	$y = $x if $y eq ''; # will pass $y == 0 if that's the case
    $g->add_edge($x, $y);
}

my %pred;

for my $aref ($g->connected_components) {
	my @items = sort {$a <=> $b} @$aref;
	my $first = $items[0];
	for my $element (@items) {
		$pred{$element} = $first;	
	}	
}

print "$_ => $pred{$_}\n" for sort {$a <=> $b} keys %pred;

__DATA__
567;456
456;345
345;234
234;123
339;228
228;117
131;
435;324
324;213
372;