sub dyn_replace {
  my ($replace) = @_;
  my @groups;
  {
    no strict 'refs';
    $groups[$_] = $$_ for 1 .. $#-;      # the size of @- tells us the number of capturing groups
  }
  $replace =~ s/\$(\d+)/$groups[$1]/g;
  return $replace;
}

##</code><code>##

my $pat = qr/foo(\d+)bar(\d+)/;
my $rep = 'x$1y$2';
my $str = 'foo36bar47';
$str =~ s/$pat/dyn_replace($rep)/e;
print "$str\n";