$_ = my $original = "/one/two///three/four/..//fourX/five////six/seven";

# you can do job 1 alone like this:

tr!/!/!s;
print "1: $original -> $_\n";

# OR you can do jobs 1 and 2 together like this;

my @srcpaths = split m!/+!, $original;
my @outpaths = ();
for my $p ( @srcpaths ) {
    if ( $p eq '..' ) {
        pop @outpaths;
    } else {
        push @outpaths, $p;
    }
}
$_ = join( "/", @outpaths );

print "2: $original -> $_\n";

##</code><code>##

    pop @outpaths or die "Bad path: $original\n";

##</code><code>##

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;