I'm uncertain how to get by your problem without turning off strict/warnings for that issue locally in your scope. Obviously that constant is defined in the Win32 module that's not being loaded on the mac. You can get around the issue like so:

if ($OS =~ m/win32/i) {
    my $attrib;
    Win32::File::GetAttributes($http_rec_localdir, $attrib);

    no strict qw(subs);
    no warnings qw(bareword);

    if ($attrib & (Win32::File::SYSTEM | Win32::File::HIDDEN)) {
        &error_dialog( "Destination Directory",
               "Destination directory is not writable." );
        return;
    }
}
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This will allow the code to run. Be careful with this kind of approach as it can lead to silent failures. You have a tight simple scope here, so it should have minimal impact.

Careful turning off strict or warnings anywhere in your code and make very certain those cases are tightly scoped. If you can make the code run without doing so, that's probably the right thing to do. Once in a while you bump into those situations that the capability was created for though.