The wfits documentation also lists an array-ref variant for the arguments.

What happens if you try that?

$x->wfits('bar.fits', [16]);
[download]

Update:

But now I look at the code, that will not work.

Try adding a hash ref as the third argument.

$x->wfits('bar.fits', 16, {});
[download]

And maybe file a bug with PDL. https://github.com/PDLPorters/pdl