my $re = qr/$this->{mask_start}\Q$key\E(.*?)$this->{mask_end}(.*?)$this->{mask_start_close}\Q$key\E$this->{mask_end_close}/is; 

while ($this->{template} =~ /$re/) { 
  my $params = $this->mask_block_params($1, $2); 
  my $html = &$callback($key, $params); 
  $this->{template} = $` . $html . $'; 
}