use Fcntl;

sub writable {
    my $fh = shift;
    (O_WRONLY | O_RDWR) & fcntl( $fh, F_GETFL, my $slush);
}

for (qw/ > < >> +< +> /) {
    open my $fh, $_, 'foo' or warn $! and next;
    print
        "$_\t",
        writable($fh)? 'writable.': 'not writable.',
        $/;
}

__END__
>       writable.
<       not writable.
>>      writable.
+<      writable.
+>      writable.