How about this: 
use strict;
use warnings;

my %pred;

while (<DATA>) {
    chomp;
    my @items = split /-/, $_;
    my $first = $items[0];
    for my $element (@items) {
        $pred{$element} = $first;
    }
}

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

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

[download]
Output: 
$ perl  predecessor.pl
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]

In reply to Re: Looking for the first item in the chain by Laurent_R
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.