package My::Module;
use strict;
sub import {
    my ($class, %args) = @_;
    ...
    # Dynamically load either My::Module::API1 or My::Module::API2
};

##</code><code>##

package My::Module::API1;
# The old API

##</code><code>##

package My::Module::API2;
# The new API

##</code><code>##

use My::Module api => 'API1';
...