#!/usr/bin/perl -w
use strict;
use warnings;
use WWW::Mechanize;
use HTML::TreeBuilder;
use List::Util qw(max);

...

my $site = "http://www.hdwallpapers.in
           /planes-desktop-  wallpapers.html";
$mech->get($site);
my @links = map {$_->text} grep {$_->text =~ m/\d+/}
$mech->find_all_links();
print max @links;