Even thinking about having a few unused entries in the symbol table is a ridiculous micro-optimisation.

If you want to keep the code clean and in one file, then I suggest something like this ...

use Devel::CheckOS;
setup_for_platform();

... application code goes here ...

# decide what platform-specific code to use
sub setup_for_platform {
  if(os_is('MicrosoftWindows')) {
    *do_stuff = \&do_stuff_for_windows;
  } elsif(os_is('Unix')) {
    *do_stuff = \&do_stuff_for_unix;
  } else {
    die("Don't know about $^O\n");
  }
}

# platform-specific function implementations
sub do_stuff_for_unix { ... }
sub do_stuff_for_windows { ... }
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