Well, taking your question literally, it's simply:

$class_name = "Calvin::Hobbes"; # from some value above
my $new_object = $class_name->new;
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There's no magic involved in using a string as a classname.

But perhaps you also intend to dynamically load a require file. In which case, I'd do something like this:

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;
}
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Notice the validation there. Essential. Without that, damage will result. Use whatever mechanism you want, but it must match one of the names you are expecting, and certainly not one of the names you are not.

Also, this approach presumes a constant constructor called new. That's not necessarily the case, since Perl doesn't require any constructor called new to be defined. In that case, you'll want a table, like:

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();
}
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The nice thing about this approach is that the table of "how" is automatically the validation table for "permitted".

-- Randal L. Schwartz, Perl hacker