$ perl -wE'
   sub f {
      my $x = $_[0];
      "" =~ /(??{ say $x; "" })/;
   }
   f("abc");
   f("def");
'
Variable "$x" will not stay shared at (re_eval 1) line 1.
abc
abc

##</code><code>##

$ perl -wE'
   sub f {
      local our $x = $_[0];
      "" =~ /(??{ say $x; "" })/;
   }
   f("abc");
   f("def");
'
abc
def