# open the text database
    unless(open(PFD,$db))
    {
        &printError("Could not open user database");
        exit(1);
    }
    my @users;
# first check if user exist
    $path='';
    while (<PFD>)
    {
        chomp;
        ($u,$p,$path)=split('\|',$_);###$u loop?? how?
        push @users, $u;
        if ($userid eq $u)
        {
            $rc=1;
            last;

        }
    }
    close(PFD);

    print $query->popup_menu(-name=>'userid',
                            -values=>\@users);
[download]

That should work, but in your code you do a last if $userid eq $u, which means @users will store all the users in the list until the one that matches $userid... not sure if that is what you want.
                - Ant