How about

$this->{ template } =~ s/$re/$this->replacement( $callback, $key, $1, 
+$2 )/eg;
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where

sub replacement {
  my $this = shift;
  my ( $cb, $key, $first, $second ) = @_;
  return
    $cb->( $key, $this->mask_block_params( $first, $second ) );
}
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I don't know if this is any faster than what you have, but at least it avoids the use of evil variables.