It's certainly a common enough error to warrant a warning (which, unsurprisingly, perl5 does give once the syntax error-producing [] is removed). But it's a only a warning, so that people with a whitespacy coding style can turn it off.
Well, it's judgement call I have to make, but I think if you're going to go to the trouble of turning off a warning, it's just as easy to install a syntax warping pragma that
lets you program all whitespacey without a warning. Meanwhile, for the vast majority of nonwhitespacey folks, Just Works beats a warning most any day, not to mention saving a whole lot of wear and tear on the FAQ.
I'm concerned that the syntax will be complex enough that introducing such a pragma without undue effort will be difficult. Perhaps you can keep it in mind when thinking about how "syntax warping pragma"s will work.