Saintmike, first of all thank you for bringing up this excellent challenge. As the answers of other monks did not seem to provide a concise answer I decided to dig into the issue to learn why. Sure enough Perl must have acted consistent as usual?

It turns out that your assumption is not correct. In both scenarios foo() result is used in list context. This snippet of code will demonstrate it: 

#!/usr/bin/perl -w
use strict;

sub foo {
   return (10,11,12,13);
}

my $ret = 0;
my $ret2= 0;
# EXPRESSION A
($ret,$ret2)||= foo();
print "ret=$ret  ret2=$ret2\n";
# ret2 returns 13, last element of list
# ret remains unused  (warning raised)

$ret=0;
# EXPRESSION B
($ret) = ($ret) || foo();
print "ret=$ret\n";
# returns 10,  first element of list

[download]

What drives the whole problem (and solution) is that the '||' operator returns the last value evaluated. And that's where the catch is!
Expression A evaluates the last element of ($ret,$ret2) against the last element of what foo() returns. Since what is listed on the lefthand of the '=' is part of the expression, only the last element of the left-hand operand gets updated.
In Expression B first Perl evaluates the expression on the righthand of the '='. The last values of the (righthand) operands are compared and because $ret is zero this option is discarded and the next operand, foo() is taken. So the result of the expression is foo's list (yes, all elements!). Subsequently this result gets assigned to what is on the lefthand of the '='.

All this shows that it can be tricky to use the '||' operator in a list context. The net result is not obvious and probably one wants to surround any such expression with a little bit of inline documentation so ease code maintenance. Frankly, for myself I would stay away from the construct where possible (except maybe in a nice JAPH).

Update: 'YAPH'=~s/Y/J/

In reply to Re: Why are "a ||= b" and "a = a || b" different? by varian
in thread Why are "a ||= b" and "a = a || b" different? by saintmike

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.