Any chance of a require? I do like it over use base even if the only thing changing between the two is that you can say isa Bar instead of use base "Bar". There is one slight catch, though:

#!/usr/bin/perl -wl
our @ISA;
use isa;

isa Foo,Bar; # drops Bar and also spits out warning...
             # works same if you quote Bar, dies if Foo
             # is quoted or we're under strict
print "@ISA";
@ISA = ();

Foo->isa("Bar"); # this works but the syntax is...funky
print "@ISA";
@ISA = ();

UNIVERSAL::isa(qw(Foo Bar)); # kinda long, eh?
print "@ISA";
@ISA = ();

main->isa(qw(Foo Bar)); # makes sense... pushes itself onto
                        # @ISA, though

__END__
Useless use of a constant in void context at isa.pl line 5.
Foo
Foo Bar
Foo Bar
main Foo Bar
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It doesn't do the right thing if you'd like to declare it for more than one package at a time. With base you can do as many as you want. The only ways I can think of fixing it is to actually export a version to the package that's using isa's functionality. I dunno. Works great for one package but otherwise doesn't seem to be as clean of a solution as it could be.