while ( my $pass = $next_pw->() ) {
    if ( unlock( $pass ) ) {
        print "$pass\n";
        last;
    }
}
sub fix_size_perm {
    my ($size, @list) = @_;
    my @curr = (0) x ($size - 1);
    push @curr, -1;
    return sub {
        if ( (join '', map { $list[ $_ ] } @curr) eq $list[ -1 ] x @curr ) {
            @curr = (0) x (@curr + 1);
        }
        else {
            my $pos = @curr;
            while ( --$pos > -1 ) {
                ++$curr[ $pos ], last if $curr[ $pos ] < $#list;
                $curr[ $pos ] = 0;
            }
        }
        return undef if @curr > $size;
        return join '', map { $list[ $_ ] } @curr;
    };
}
sub unlock { $_[0] eq 'john' }