package Unsigned;
use strict;
use warnings;
use Scalar::Util qw( looks_like_number );
use Carp qw( croak );

sub TIESCALAR {
    my ($class, $number) = @_;

    my $self = bless \$number, $class;
    $self->_check_number;

    return $self;
}

sub _check_number {
    my ($self, $number) = @_;

    my $checkvalue = defined $number ? $number : $$self;
    croak "Sie müssen eine Nummer eingeben" if not looks_like_number($checkvalue);
    croak "Nur Positive Nummer erlaubt"     if     $checkvalue < 0;
}

sub FETCH { ${$_[0]} }
sub STORE {
    my ($self, $newvalue) = @_;

    $self->_check_number($newvalue);
    $$self = $newvalue;
}

1;

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

tie my $wert, 'Unsigned', 10;
print $wert, "\n";

$wert = 100;
print $wert, "\n";

$wert = -100;
print $wert, "\n";

$wert = 'hallo';
print $wert, "\n";

10
100
Nur Positive Nummer erlaubt at ./Unsigned.pl line 12