#!/usr/bin/perl -w
use strict;
use warnings;

my %test = (
    foo1    => 'a',
    foo2    => 'b',
    foo10   => 'c',
);

my ($letzter) = sort {substr ($b,3) <=> substr ($a,3)} grep {$_ =~ /^foo\d+$/} (keys %test);
print "$letzter\n";