C:\>perl -MScalar::Util="looks_like_number" -wle "$r = ''; $x = \$r; p
+rint 'lln' if looks_like_number($x); $x += 1"

C:\>perl -MScalar::Util="looks_like_number" -wle "$x = 'hello'; print 
+'lln' if looks_like_number($x); $x += 1"
Argument "hello" isn't numeric in addition (+) at -e line 1.

C:\>
[download]

$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 at˛, 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.)

˛ 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

perlnumber?

numify?