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>perl -MO=Concise -e"\substr($_,1)" 2>&1 | find "substr"
5           <@> substr[t4] sKRM/2 ->6
...
                          ^
                          |
                          Not modifiable => no magic created.

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 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

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

--------------------
20,000
...
substr3 0.445/s      --    -98%    -99%  (warning: unreliable count)
s///     23.4/s   5162%      --    -42%
substr4  40.1/s   8897%     71%      --

use strict;
use warnings;
...
   cmpthese($OUTER, \%bench);
   print("\n");
}