Being new to perl, please dont flame me.
But, why cant you store the string in reverse in another string, and then work forwards...
Eg.

my($str) = "abcdefgh";
my($str_rev);

print "before $str\n";
$str_rev = reverse($str);
print "after $str_rev \n";
[download]

This sets str_rev to "hgfedcba".