UNIVERSAL::isa - Attempt to recover from people calling UNIVERSAL::isa as a function


version 1.20171012


    # from the shell
    echo 'export PERL5OPT=-MUNIVERSAL::isa' >> /etc/profile

    # within your program
    use UNIVERSAL::isa;

    # enable warnings for all dodgy uses of UNIVERSAL::isa
    use UNIVERSAL::isa 'verbose';


Whenever you use "isa" in UNIVERSAL as a function, a kitten using Test::MockObject dies. Normally, the kittens would be helpless, but if they use UNIVERSAL::isa (the module whose docs you are reading), the kittens can live long and prosper.

This module replaces UNIVERSAL::isa with a version that makes sure that, when called as a function on objects which override isa, isa will call the appropriate method on those objects

In all other cases, the real UNIVERSAL::isa gets called directly.

NOTE: You should use this module only for debugging purposes. It does not belong as a dependency in running code.



This sub contains the definition of the original UNIVERSAL::isa definition, in case you need it.


If the lexical warnings pragma is available, this module will emit a warning for each naughty invocation of UNIVERSAL::isa. Silence these warnings by saying:

    no warnings 'UNIVERSAL::isa';

in the lexical scope of the naughty code.

After version 1.00, warnings only appear when naughty code calls UNIVERSAL::isa() as a function on an invocant for which there is an overridden isa(). These are really truly active bugs, and you should fix them rather than relying on this module to find them.

To get warnings for all potentially dangerous uses of UNIVERSAL::isa() as a function, not a method (that is, for all uses of the method as a function, which are latent bugs, if not bugs that will break your code as it exists now), pass the verbose flag when using the module. This can generate many extra warnings, but they're more specific as to the actual wrong practice and they usually suggest proper fixes.



UNIVERSAL::can for another discussion of the problem at hand.

Test::MockObject for one example of a module that really needs to override isa().

Any decent explanation of OO to understand why calling methods as functions is a staggeringly bad idea.


Bugs may be submitted through the RT bug tracker (or


  • Audrey Tang <>

  • chromatic <>

  • יובל קוג'מן (Yuval Kogman) <>


  • Karen Etheridge <>

  • Graham Knop <>

  • Ricardo Signes <>


This software is copyright (c) 2011 by

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.