my $n = unpack("v", $str);
     $str = substr($str, 1); 
##</code><code>##

$ time perl -e '$a = "a" x 20_000; $a = substr($a, 1) while length($a);'
user    0m0.350s

$ time perl -e '$a = "a" x 40_000; $a = substr($a, 1) while length($a);'
user    0m0.791s

$ time perl -e '$a = "a" x 80_000; $a = substr($a, 1) while length($a);'
user    0m2.683s
##</code><code>##

     my $n = unpack("v", substr($str, $off, 2));
     $off += 2;
##</code><code>##

    my $n = unpack("v", substr($str, $off, 2, ""));