It would be interesting to know which behavior you rather wanted/expected.

If it's just a no-op, try triple semi-colons as visual indicator

;;;

or a comment

#TODO yadda yadda

If you want a warning instead of a die you could define your own sub __todo__

use strict;
use warnings;
use Data::Dump qw/pp dd/;

package BLA;

# poor mans import
BEGIN {
    *__todo__ = \&TODO::__todo__;
}



sub add {
    my ( $arg1,$arg2 ) = @_ ;

    __todo__ "check argument type";

    return $arg1+$arg2;
}

add 1,2;


# other module
package TODO;
use Carp;

sub __todo__ {

    carp "TODO: $_[0]";
}
[download]

TODO: check argument type at d:/tmp/pm/t_carp.pl line 17.
[download]

update

improved demo with packages