You could do something like

#! perl -slw
use strict;
use warnings;

package xtest::mac;
sub get_answer{ 'Apple' }
package xtest::sun;
sub get_answer{ 'Sun' }
package xtest::aix;
sub get_answer{ 'IBM' }
package xtest::win;
sub get_answer{ 'Win' }
package xtest;
if( $^O =~ m[(dar|sol|aix|win)]i ) {
     ## Export to main for testing only. 
     *main::get_answer = \&{"xtest::\L$1\E::get_answer"};
}
else {
    die 'Unknown OS';
}
1;
[download]

Then

P:\test>perl -Mxtest -e"print get_answer()"
Win
[download]

If you have lots of vaient subs, then you can put the platform specific packages in separate modules, use the standard export mechanism and require them from the base package and then call test::$1::export(); to get the exports into your package.