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use Devel::Symdump;

...
foreach (Devel::Symdump->rnew->packages) {
    push @subclasses, $_ if UNIVERSAL::isa($_, $class);
};

sub subclasses_of {
  my $class = shift;
  grep { $_->isa($class) && $_ ne $class} Devel::Symdump->rnew->packag
+es;
};