What does wantarray have to do with it? All of the following return something different based on context:

• sub { ...; @foo }
• sub { ...; ($x,$y) }
• sub { ...; /(...)/ }
• sub { ...; grep { ... } @x }
• and I could go on and on and on and on

What about

sub foo { ...; bar() }
[download]

The whole "independent of parameters" bit is problematic too.

sub { ...; if ($_[0]) { $foo } else { @bar } }
[download]

It might be possible to identify some subs that only return a scalar by poking at the opcode tree, but for many subs, it probably would be quite hard to tell.