If you want it to be portable, abstract it one level, and then bring in whatever code you need based on the OS in your abstraction layer rather than the application layer. As you need support for new environments, you only adjust your abstraction layer.

Example:

# Application.pl

use IO::File::AsUser;       # Your abstraction layer
my $fh = IO::File::AsUser->open($file, $user);

# AsUser.pm

package IO::File::AsUser;

#blah blah
sub open {
    if ($Windows) {
        do_what_needs_to_be_done_on_windows();
    } elsif ($Unix} {
        do_what_needs_to_be_done_on_unix();
    }
}
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