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 ... using it three times in my highest rated node Saturn I see. :)

See also:

• Perl Secret Operators (catonmat)
• Re: Hidden features of Perl (More Secret Operator References) (describes Perl secret operators)
• perlsecret on CPAN by BooK
• BooK proposes a new Perl secret operator (Secret Operator References)
• The Lighter Side of Perl Culture (Part IV): Golf - the inchworm ~~ secret operator is mentioned as a standard Perl golfing technique (inventor unknown)
• Re: how to force scalar context (without scalar())? - mentions inchworm as an alternative to scalar

Note that the inchworm secret operator should not be confused with Perl's experimental smartmatch operator.

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).

No cause for pause. undef is promoted to the empty string, used as a hash key and assigned undef as its value because that's the default value of items in the empty list | used by Perl when a key in a hash assignment list has no corresponding value, or Odd number of elements in hash assignment.

Win8 Strawberry 5.8.9.5 (32)  Sat 08/07/2021 23:15:45
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Data::Dump qw(dd);

my %h = undef;
print scalar %h, "\n";

dd \%h;

^Z
Odd number of elements in hash assignment at - line 7.
Use of uninitialized value in list assignment at - line 7.
1/8
{ "" => undef }
[download]

Update: Clarified (I hope!) the explanation of odd-item hash assignment.