I don't understand why you want to act in such an obscure manner, because without the /m modifier, as here, there is no difference between /^\s*/ and /\A\s*/. So you might just as well use the far more familiar form, with the caret.

The difference between /\Z/, /\z/ and /$/ is far more significant.

Well, I do agree with you on the RTFM part... it's all in there. Looking there should become a more natural response, than asking here.

I'd prefer

  s/^\s+//;
[download]

anyway, as this wouldn't do an empty substitution if the string doesn't start with whitespace.