perlsyn says that it only uses the 'smart match both sides of OR/AND/||/&&' _if_ both sides are prone to smart matching, i.e., they do not contain """
. a subroutine or method call
. a regular expression match, i.e. /REGEX/ or $foo =~ /REGEX/, or a negated regular expression match $foo !~ /REGEX/
. a comparison such as $_ < 10 or $x eq "abc" (or of course $_ ~~ $c)
. defined(...), exists(...), or eof(...)
. A negated expression !(...) or not (...), or a logical exclusive-or (...) xor (...)
"""