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

my %hash = (
        foo1    => 'test',
        foo2    => 'test2',
);

for my $k (keys %hash) {
    next unless( my ($id) = $k =~ /^foo(\d+)$/ );
    # $id entspricht $1 des regex matchings

    print $id, "\n";
}