package XPlat;
use strict;
my $OS_TYPE = $^O eq 'MSWin32' ? 'win' :
              ($^O eq 'nonstop_kernel' ? 'nsk' : 'unix');
push @ISA, "XPlat::$OS_TYPE";

sub XPlat::win::fn  { print "windows version of function\n" }
sub XPlat::nsk::fn  { print "nsk version of function\n"     }
sub XPlat::unix::fn { print "unix version of function\n"    }

package main;

XPlat->fn(); # calls function in whichever package is appropriate