if ($test =~m#(??{substr($test, 0, 8)})#) {  }

##</code><code>##

use strict;
use warnings;

my $test = "fee fi fo fee";
$_++;

if ($test =~ m!((??{ substr($test,0,$_) }))!) { };


##</code><code>##


use strict;
use warnings;

my $test = "fee fi fo free";

if ($test =~ m!((??{ print "$_\n";substr($test,0,$_) }))!) {
};


##</code><code>##

fee fi fo free
Argument "fee fi fo free" isn't numeric in substr at (re_eval 1) line 1.