use Devel::Sub::Which qw/:universal/;

INIT { # after all modules have been loaded. Will break with mod_perl

    my $which = __PACKAGE__->which("foo");

    ( my $class = $which ) =~ s/::[^:]+$//;

    die "Subclassing 'foo' is not allowed" unless $class eq __PACKAGE_
+_;
}
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Technicalities aside - are you sure you really want to check this? I can't imagine a scenario where this would be a good thing to do.