perl -e '$a="bla"; print sprintf "%s %p %s",$a,$a,\$a;'
bla 8171078 SCALAR(0x8171078)
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Note that in the original complaint:

since it doesn't even match the address of a variable's value when passed to a 'C' XS module??

I'd assume that the value passed to a 'C' XS module is most likely to be the PV (pointer to the first byte of the string value) of the SV, not the address of the SV itself (a Perl-specific data structure that defines a scalar value). I'm pretty sure that 0+\$x and ''.\$x report the address of the SV. Your testing shows that %p reports the same value. So, although the original author didn't really pin down what value they are passing to their C code, I wouldn't be surprised if they don't match for the original author.

If you want to get at the address of the PV in an SV from pure Perl, you can use some old, nasty tricks of mine or a new, shiny, Perl-only module or just use someone else's C-code module (I was going to include a link for this one but I couldn't remember which B:: module I'd heard mentioned or even if it was something more like Scalar::Util or Scalar::Utils or Devel::Peek -- maybe several of those would work?).

- tye        

%p was there before \ (references). It was useful in debugging output as a quick way to match up variables between different lines of output. C's printf had %p, so why shouldn't Perl have copied it over along with the rest of them?