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

my %hash = (
	100 => 'foo',
	200 => 'bar',
	300 => 'foobar',
	400 => 'foofoo',
	500 => 'barbar',
);

my $test = '300';
my $nr = grep {$_ eq $test} keys %hash;
print "Laufende Nr. $nr\n";