# 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();
    }
}