use strict;
use warnings;

my $test = "fee fi fo free";

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


##</code><code>##

 DB<1> V main          # $_ never appears after this command
 DB<2> s
main::(bah4.pl:6):      if ($test =~ m!((??{ substr($test,0,$_) }))!) {};
 DB<2> V main          # $_ never appears after this command
 DB<3> s
main::((reeval 2)[bah4.pl:6]:1):
 DB<3> V main    #At this point again variables in main are 
                 #listed there is a line: 
                 #  $_ = 'fee fi fo free'
  DB<4> s
Argument "fee fi fo free" isn't numeric in substr at (reeval 2)[bah4.pl:6] line 1.