I'll give you merlyn's "don't do my $class = ref($obj) || $obj" advice. Don't do it.

When I need abstract methods, I usually do:

sub some_abstract_method {
  __abstract($_[0], caller);
  # ...
}


sub __abstract {
  my ($thing, $pkg, $file, $line) = @_;
  return if $thing ne __PACKAGE__;
  my $func = (caller 1)[3];
  die "$func is abstract; in $file line $line\n";
}
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Same general principle, but I think it's a little cleaner.