my $old = \&IWL::Script::setScript;
*IWL::Script::setScript = sub {
  ...;
  goto &$old;
};

##</code><code>##

# Untested.

use Scalar::Util 'reftype';
use 5.010; # For ~~.

my $old = \&IWL::Script::setScript;
*setScript = sub {
  my ($self, $param) = @_;

  $param = $p2js->covert($param)
    if reftype($param) ~~ 'CODE';

  @_ = ($self, $param);

  # or:
  #splice @_, 1, 1, $p2js->convert($param)
  #  if reftype($param) ~~ 'CODE';

  goto &$old;
};