I originally wrote "evaled" and changed it to avoid confusion with string eval.

But actually it is first evaled¹ and then of course stringified.

It's a subexpression returning a value, even within interpolation:

  DB<119> %h=(1=>42)
 => (1, 42)

  DB<120> $a=2
 => 2

  DB<121> "<<< $h{3-$a} >>>"
 => "<<< 42 >>>"
[download]

so 3-$a is of course executed.

> Thanks for raising some interesting distinctions.

I took your "?" as a question. :)

May sound like nitpicking, but I'm convinced Perl's behavior needs a more axiomatic (mathematical) explanation.

Otherwise people rely far to often on try-and-error coding and consequently cargo cult, believing that the interpolation on the RHS of s/// is just one other magic DWIM mechanism.

Even more important if one tries understanding what s///e and s///ee do.

That's why I once wrote:

s/RegEx/substitutions/: Variable interpolation and when to use /e - modifiers

¹) think block eval or do