I'm not sure what code you're using for this. The node you reference doesn't do what your node seems to be doing.

Anyway, I wrote this to verify the behavior you describe:

use Benchmark 'cmpthese';

my $str = 'a' x 1_000_000;

cmpthese( -2, { substr_lvalue => sub { substr_lvalue( "$str" ) },
                substr_mod    => sub { substr_mod( "$str" ) },
          }
    );

sub substr_lvalue {
    substr( $_[0], 0, 1 ) = '' while $_[0];
    return;
}

sub substr_mod {
    substr( $_[0], 0, 1, '' ) while $_[0];
    return;
}
[download]

                Rate substr_lvalue    substr_mod
substr_lvalue 2.58/s            --          -50%
substr_mod    5.16/s          101%            --
[download]

I don't have much to add beyond that. My first thought about this was that the lvalue case must allow something that the four argument case can't allow, but I hadn't figured out what that was when I read the reply from tye that laid it out.