$x = \$r;

I am confused why you're taking this step? This means that $x is a reference, and a reference is like a dual-valued variable*: As a string, it's "SCALAR(0xabc)", which I would guess is what looks_like_number is looking at², and in numeric context, it's the memory address, which is why $x += 1 doesn't warn.

* Update: Triple-valued? It's a reference, string, and number ;-) (Or rather: It's a reference, that gets converted to different values depending on context.)

² Update 2: Hmmm, nope, looks like it's checking the flags in this case.

I am confused why you're taking this step?

Nothing insidious or profound.
I read "this gives you the exact internal function that Perl uses to check strings and generate that warning in the first place" as implying both that:

1) if looks_like_number($x) returns true, then there will be no warning given when $x is used in numeric context;
&&
2) if looks_like_number($x) returns false, then a "non-numeric" warning will be given when $x is used in numeric context.

I find it interesting that the latter of the two is not necessarily true - furthermore, I found it so interesting that I decided to provide an example where looks_like_number($x) returns false, yet no warning is issued when $x is used in numeric context.

Your second update references the use of looks_like_number() in sv.c.
Note that Scalar::Util::looks_like_number() is a different function. (That is, it doesn't simply wrap the perl API function of the same name ... it does some other stuff as well.)

Cheers,
Rob

Yes, I guess I was a little too unspecific* in what I wrote: Given a string $x, looks_like_number($x) should accurately report whether the warning will occur.

Note that Scalar::Util::looks_like_number() is a different function. (That is, it doesn't simply wrap the perl API function of the same name ... it does some other stuff as well.)

True, it's a different function - AFAICT, Scalar::Util::looks_like_number just adds get-magic to Perl_looks_like_number in sv.c, which in turn calls Perl_grok_number in numeric.c, if the scalar has a valid string component (SvPOK). And my understanding is that grok_number is the same internal function that is used to inspect strings for their numeric content - see e.g. Perl_sv_2nv_flags in sv.c, which is called by the SvNV macro, and which calls not_a_number to generate the "Argument isn't numeric" warning.

I had done all this research in the past, and perhaps remembered it a little too simplified, thanks for poking me on this :-)

* Update: Well, to nitpick: I did write "the exact internal function that Perl uses to check strings", and you didn't give it a string ;-P But "exact" is indeed inaccurate.