OK, once I knew what the answer was... it helped me find the answer in a manual. From 'Programming Perl, 2nd Edition', pp 92-93. It lists the assignment operators, including = and ||=, and says

List assignment may be done only with the plain assignment operator, =
+.  In a list context, list assignment returns the list of new values 
+just as scalar assignment does. ...
[download]

Hitting your thumb with a hammer - there's more than one way to do it!