use Devel::Symdump;

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

##</code><code>##

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