I found a solution!

Before

-textvariable =>$mockOb->search

sub search{
    my $self = shift();
    if(@_) {
        $self->{search} = shift();
        $self->{search} =~ s/^\s+|\s+$//g;
    }
    return $self->{search};
}
[download]

After

-textvariable =>$mockOb->searchRef

sub searchRef{
    my $self = shift();
    
    if(@_) {
        $self->{search} = shift();
        $self->{search} =~ s/^\s+|\s+$//g;
    }
    
    return \$self->{handelspartnerSuche};
}
[download]

I do not understand the solution completely, but it seems to work just fine.