Borrowing from Laurent_R's answer, you might want to consider using Graph::Undirected to get the connections. I also borrowed this graph example from somewhere, probably from here at Perl Monks but can't find the original in a search. 
#!/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;

[download]
Prints: 
117 => 117
123 => 123
131 => 131
213 => 213
228 => 117
234 => 123
324 => 213
339 => 117
345 => 123
372 => 372
435 => 213
456 => 123
567 => 123

[download]
Update: Changed $g->add_edge($x, $y || $x); to $g->add_edge($x, $y); and added $y = $x if $y eq ''; so to allow $y to be a possible zero.

In reply to Re: Looking for the first item in the chain by Cristoforo
in thread Looking for the first item in the chain by vagabonding electron

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.