$class_name = "Calvin::Hobbes"; # from some value above
my $new_object = $class_name->new;

##</code><code>##

if (grep $class_name eq $_, qw(Calvin::Hobbes Peter::Gabriel Dog::Day::Afternoon)) {
  # must validate the name, or security holes will run rampant
  eval "require $class_name"; die $@ if $@;
  $object = $class_name->new;
}

##</code><code>##

my %make_a = qw(
  Calvin::Hobbes new
  Peter::Gabriel play
  Dog::Day::Afternoon watch
);
...
if (my $constructor = $make_a{$class_name}) {
  unless ($INC{$class_name}) {
    eval "require $class_name"; die $@ if $@;
  }
  $object = $class_name->$constructor();
}