ANS: $c,$f,$g,$h,$j, and $m are used it ways that trigger warnings
 
$a++;          # this is ok
$b += 1;       # this is ok
$c = $c + 1;   # but this is bad???

$d->{size} = 'big';   # both are just fine... treating undef as a hash ref 
$e->[5] = 35;         # is ok, but treat it like a 0 and I get yelled at?

$_ .= "$f";      # bad
$_ .= $g;        # still bad
$_ = 'h=' . $h;  # bad.... ok so you can't treat undef like a string
$_ = $i . '=i';  # wait, no warning, maybe you can
$_ = "j=$j";     # nope this tosses an error
$_ = "$k=k";     # but this doesn't.... huh?

$_ = $l x 5;     # neither does this
$_ = "$m" x 5;   # but if I do the stringifing myself I get yelled at