I think you mostly want that so you can check whether something can influence how a class behaves, which seems to point mostly to @ISA to me (barring weirdo UNIVERSAL or MRO trickery). If you want to check whether @Some::Package::ISA exists, you can look into the namespace hash:

#!perl -w
use Data::Dumper;

sub isa_exists {
    my ($package) = @_;
    exists ${"$package\::"}{ISA} and defined *{${"$package\::"}{ISA}}{
+ARRAY};
};

print "Now you don't\n";
print isa_exists('Some::Package'),"\n";

eval q(package Some::Package;@ISA='foo';);
print "Now you see it\n";
print isa_exists('Some::Package'),"\n";

print "Now you don't\n";
print isa_exists('Some::Other::Package'),"\n";
print "Now you don't\n";
print isa_exists('Some::Other::Package'),"\n";


eval q(require A::Package::That::Doesn't::Exist);
print "A::Package::That::Doesn't::Exist ($@)\n";
print isa_exists("A::Package::That::Doesn't::Exist");
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