same warning multiple times when package in #ISA is not loaded - perl

Question

while refactoring some of my perl code i
noticed the following strang behaviour.
consider this small sample script:
#!/usr/bin/perl -w
package test;
use strict;
my $obj = bless( {}, __PACKAGE__);
our #ISA = qw( missing );
exit(0)
the expected warning
Can't locate package missing for #test::ISA at test.pl line 8
appears three times instead of only one time. what are the other two triggers for this warning? all three refer to exit(0).
perl-version 5.12.4 on gentoo linux.
thanks.

Answer 1

I believe tjd got the answer. If you add a DESTROY() and AUTOLOAD() method to the test package, you will get only one warning (about the absence of #test::ISA).
package test;
use strict;
sub DESTROY {}
sub AUTOLOAD {}
You generally need to make sure that the packages listed in the #ISA list have already been loaded. In your example, you might have expected to see something like:
package test;
use strict;
use missing;
our #ISA = ('missing');
It is a bit curious that your package doesn't have an explicit new() method. Instead, you have a statement that calls bless();
If you had a new() method, like this:
sub new() { return bless {}, __PACKAGE__; }
Then you would not see the triple error message until something called new();
package main;
my $object = test->new(); # Triple warning
You might want to use the pragma 'base', which will give you a fatal error saying that it cannot load the 'missing' package:
package test;
use strict;
use base ('missing');
sub new { bless {}, __PACKAGE__);
The base pragma attempts to load the missing package for you. You do not need a separate 'use missing' statement.
So the final thing might look like this:
package test;
use strict;
use warning;
use base ('missing');
sub new { bless {}, __PACKAGE__);
sub DESTROY {}
sub AUTOLOAD {}
1;
You could then wrap this all into a file called test.pm and use it in a script:
use strict;
use test;
my $object = test->new(); # Should fatal, as there is no 'missing' module
Hope this helps.