fcntl is not supported on win32 (see perlport).

You might try using Win32API::File or simply try printing

sub _is_writable {
    my $fh = shift;
    return eval { defined print { $fh } ''; };
}
[download]

though that may not be desireable.

update: It just dawned on me that printing on a CLOSED filehandle is different error, than simply a filehandle thats not opened for writing, in which case you want to use the fileno function.