With all this talk of scalar, I had to look it up because, despite using Perl heavily for twenty years, I've never actually used it! Which reminded me that I have employed the equivalent "secret" version, namely the infamous inchworm ~~ secret operator

Just a little nitpick: the "inchworm" isn't exactly equivalent, as it doesn't pass through lvalue context, which scalar does as of 5.22.

$ perlbrew exec perl -e 'for(scalar($#foo)) { $_=3 } warn "$] ".@foo."
+\n"' >/dev/null
5.034000 4
5.032001 4
5.030003 4
5.028003 4
5.026003 4
5.024004 4
5.022004 4
5.020003 0
5.018004 0
$ perlbrew exec perl -e 'for(~~$#foo) { $_=3 } warn "$] ".@foo."\n"' >
+/dev/null
5.034000 0
5.032001 0
5.030003 0
5.028003 0
5.026003 0
5.024004 0
5.022004 0
5.020003 0
5.018004 0
[download]

Update: The example above is one of the exceptions for when ~~ is not equivalent to scalar anyway:

$ perl -le 'print scalar($#foo)'
-1
$ perl -le 'print ~~$#foo'
18446744073709551615
[download]

... but the point still stands (though interestingly, my $foo="x"; for(scalar($foo)) {$_.="y"} worked even before 5.22).

Impressive analysis!

It doesn't surprise me there are subtle differences. I only mentioned secret operators for some light relief, and (unlike BooK ;), would never inflict the inchworm secret operator on my workmates in production code ... my rule number one at work being "Correctness, simplicity and clarity come first. Avoid unnecessary cleverness".

Curiously, your second example seems to be fixed by adding use integer:

$ perl -le 'print ~~$#foo'
18446744073709551615
$ perl -Minteger -le 'print ~~$#foo'
-1
[download]

... which reminds me of this recent reply by BillKSmith ... while typing -Minteger from the command line just now reminded me of chromatic's comical -Modern::Perl command line hack. :)

Update: See also: the "Inchworm" and "Inchworm on a stick" sections at perlsecret and The Lighter Side of Perl Culture (Part IV): Golf (search for inchworm).

... chromatic's comical -Modern::Perl command line hack. :)

See also oo and ojo, plus probably a few others out there :-)