package My::Functions;
use strict;

my $OS_TYPE = $^O eq 'MSWin32' ? 'win' :
              ($^O eq 'nonstop_kernel' ? 'nsk' : 'unix');

if ($OS_TYPE eq 'win') {
  require My::Functions::Win32;
} else {
  require My::Functions::Unix;
};

1;
# ---- File My/Functions/Win32.pm:
package My::Functions::Win32;
package My::Functions;
sub foo {};

# ---- File My/Functions/Unix.pm:
package My::Functions::Unix;
package My::Functions;
sub foo {};