my @outpath = ();
for my $p ( split m!/+!, $original ) {
    if ( $p eq '..' ) {
        pop @outpath or die "Bad path: $original";
    }
    else {
        if ( @outpath ) {
            my $chk = join "/", @outpath, $p;
            die "Path not found: $chk" unless ( -d $chk );
        }
        push @outpath, $p;
    }
}
my $goodpath = join "/", @outpath;