you already received many good options and approaches, but your:

> BTW does Perl have a system variable "debug" that is global and does not need qualification with the package name?

prompts me for another answer: yes! you have %ENV at your disposal and you can create your own varaiable like DEBUG_TEMP that is uber global

#cat Example.pm

package Example;

use strict;
use warnings;

sub my_test{
        print "output from sub\n";
        print "package ",__PACKAGE__," spot debug!!\n" if $ENV{DEBUG_T
+EMP};
}


1;


#cat test-env.pl

use strict;
use warnings;

use lib '.';
use Example;

Example::my_test();     # test without the var

$ENV{DEBUG_TEMP} = 1;

Example::my_test();     # test with the var


#perl test-env.pl

output from sub         # output before var setting
output from sub         # two lines after var setting
package Example spot debug!!


# when the program exits, DEBUG_TEMP disappears

#perl -I . -MExample -e "Example::my_test()"

output from sub
[download]

L*