You empty out $ENV{PATH} at the top of your script. This is good.

But then, you expect $ENV{"PATH"} to still be available when you call system. That's not how it works.

Personally, I recommend that you hardcode the path to perl or use $^X for the path to the current interpreter:

my $perl = $^X;
system($perl, $update_location) == 0
    or die "Couldn't launch $perl: $!";
[download]