>perl -MO=Concise -e"\substr($_,1)" 2>&1 | find "substr"
5           <@> substr[t4] sKRM/2 ->6
                              ^
                              |
                             "M"odifiable => magic created.

>perl -MO=Concise -e"$a = substr($_,1)" 2>&1 | find "substr"
5        <@> substr[t3] sK/2 ->6
                          ^
                          |
                          Not modifiable => no magic created.

##</code><code>##

Pass      1: Creates a 79,999 char string & copies it into $a's buffer
Pass      2: Creates a 79,998 char string & copies it into $a's buffer
Pass      3: Creates a 79,997 char string & copies it into $a's buffer
...
Pass 79,999: Creates a      1 char string & copies it into $a's buffer
Pass 80,000: Creates a      0 char string & copies it into $a's buffer

##</code><code>##

$a =~ s/.//s;          # Remove leading char with no copying.
substr($a, 0, 1, '');  # Remove leading char with no copying.

##</code><code>##

--------------------
20,000

          Rate substr3    s/// substr4
substr3 1.61/s      --    -93%    -96%
s///    23.1/s   1336%      --    -44%
substr4 41.2/s   2460%     78%      --

--------------------
40,000

           Rate substr3    s/// substr4
substr3 0.884/s      --    -96%    -98%
s///     23.1/s   2509%      --    -45%
substr4  41.6/s   4606%     80%      --

--------------------
80,000

           Rate substr3    s/// substr4
substr3 0.445/s      --    -98%    -99%  (warning: unreliable count)
s///     23.4/s   5162%      --    -42%
substr4  40.1/s   8897%     71%      --

##</code><code>##

use strict;
use warnings;

use Benchmark qw( cmpthese );

my $OUTER = $ARGV[0] || -5;

my %tests = (
   'substr3' => '$a = substr($a, 1)',
   'substr4' => 'substr($a, 0, 1, "")',
   's///'    => '$a =~ s/.//s',
);

my %bench = map { $_ => "use strict;
                         use warnings;
                         for (1 .. our $inner) {
                            local our \$a = our \$arg;
                            $tests{$_} while length(\$a);
                         }
                        "
                }
              keys %tests;

for (
  [ '20,000', 80_000/20_000, 'a' x 20_000 ],
  [ '40,000', 80_000/40_000, 'a' x 40_000 ],
  [ '80,000', 80_000/80_000, 'a' x 80_000 ],
) {
   my $name         = $_->[0];
   local our $inner = $_->[1]
   local our $arg   = $_->[2];
   print("--------------------\n");
   print("$name\n");
   print("\n");
   cmpthese($OUTER, \%bench);
   print("\n");
}