I don't see a need for a look ahead. Try:

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

for my $test (
    'https://www.robidu.de/', 'http://www.robidu.de/',
    'https://robidu.de/',     'https://forum.robidu.de/'
    )
{
    if ($test =~ m~^https?://(?:www\.)?robidu\.de/~) {
        print "'$test' matched\n";
    } else {
        print "'$test' didn't match\n";
    }
}
[download]

Prints:

'https://www.robidu.de/' matched
'http://www.robidu.de/' matched
'https://robidu.de/' matched
'https://forum.robidu.de/' didn't match
[download]