amwl - thank you, caller() is the key, and Carp::Heavy has the detail - my problem was that I was doing it in the new() - checking caller in a sub-routine of the same package provides the answer:

package My::Factory;
1;
sub new {
  my $factory = shift;
  my $name    = shift;
  my $classfile = package_name();
  $classfile =~ s/Factory.pm$/$name.pm/;
  if ( -e $classfile ) {
    require $classfile;
    my $classname = "My::$name";
    return $classname->new(@_);
  } else {
    die "class not found: $classfile";
  }
}

sub package_name {
  my (@called) = caller(0);
  return $called[1];
}
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