with length()
        1000 L=    256000 t=0.510602
        2000 L=    512000 t=1.544809
        3000 L=    768000 t=2.558511
        4000 L=   1024000 t=3.598220
        5000 L=   1280000 t=4.622924
        6000 L=   1536000 t=5.666133
        7000 L=   1792000 t=6.634827
        8000 L=   2048000 t=7.653030
        9000 L=   2304000 t=8.687737
       10000 L=   2560000 t=9.727445
       11000 L=   2816000 t=10.728646
       12000 L=   3072000 t=11.764352
       13000 L=   3328000 t=12.804560
       14000 L=   3584000 t=13.783257
       15000 L=   3840000 t=14.836966

with a scalar
        1000 L=    256000 t=0.003712
        2000 L=    512000 t=0.003200
        3000 L=    768000 t=0.003433
        4000 L=   1024000 t=0.003232
        5000 L=   1280000 t=0.003398
        6000 L=   1536000 t=0.003669
        7000 L=   1792000 t=0.004407
        8000 L=   2048000 t=0.002218
        9000 L=   2304000 t=0.004507
       10000 L=   2560000 t=0.002192
       11000 L=   2816000 t=0.005269
       12000 L=   3072000 t=0.002203
       13000 L=   3328000 t=0.006128
       14000 L=   3584000 t=0.002311
       15000 L=   3840000 t=0.002231

##</code><code>##

use strict;
use warnings;
use feature 'say';
use feature 'state';
use utf8;
use Time::HiRes;

$|++;
my $chunk = '€' x 256;
my $td = Time::HiRes::time;
my $tf;
my $l;

say "with length()";
my $str = new LenTestA;
for my $n (1..15_000){
    state $count = 0;
    $str->add($chunk);
    $l = $str->len;

    $count++;
    if ($count % 1000 == 0){
        $tf = Time::HiRes::time;
        say sprintf "%12d L=%10d t=%f", $n, $l, $tf-$td;
        $td = $tf;
    }
}

$td = Time::HiRes::time;
say "\nwith a scalar";
$str = new LenTestB;
for my $n (1..15_000){
    state $count = 0;
    $str->add($chunk);
    $l = $str->len;

    $count++;
    if ($count % 1000 == 0){
        $tf = Time::HiRes::time;
        say sprintf "%12d L=%10d t=%f", $n, $l, $tf-$td;
        $td = $tf;
    }
}

{
    package LenTestA;
    sub new {
        my $class = shift;
        my $self = '';
        return bless \$self, $class;
    }
    sub add {
        my ($self, $data) = @_;
        $$self .= $data;
    }
    sub len {
        my $self = shift;
        return length $$self;
    }
}
{
    package LenTestB;
    my $len;
    sub new {
        my $class = shift;
        my $self = '';
        return bless \$self, $class;
    }
    sub add {
        my ($self, $data) = @_;
        $$self .= $data;
        $len += length($data);
    }
    sub len {
        my $self = shift;
        return $len;
    }
}