Overloading regexp issue with lexical scope

[atoomic]

extract from re/overload.t

 {

     # if the pattern gets silently re-parsed, ensure that any eval'ed
     # code blocks get the correct lexical scope. The overloading of
     # concat, along with the modification of the text of the code block,
     # ensures that it has to be re-compiled.

     {
     package OL_MOD;
     use overload
         q{""} => sub { my ($pat) = @_; $pat->[0] },
         q{.}  => sub {
                 my ($a1, $a2) = @_;
                 $a1 = $a1->[0] if ref $a1;
                 $a2 = $a2->[0] if ref $a2;
                 my $s = "$a1$a2";
                 $s =~ s/x_var/y_var/;
                 bless [ $s ];
              },
     ;
     }

     BEGIN {
      overload::constant qr => sub { bless [ $_[0] ], 'OL_MOD' };
     }

     $::x_var  = "ABC";
     my $x_var = "abc";

     $::y_var  = "XYZ";
     my $y_var    = "xyz";

     use re 'eval';
     my $a = 'a';
     print qq[ok - 1\n] if "xyz"  =~ m{^(??{ $x_var })$}; # OL_MOD
     print qq[ok - 2\n] if "xyza" =~ m{^(??{ $x_var })$a$}; # OL_MOD runtime"
 }

The runtime test is working but not the first one

HEAD=d7aa198ab3b6adacbee32be33e189976e495be60