... I'm normally quite prodigious with my horizontal whitespace ...

Either prodigality or parsimony seem to work; e.g.,  print $n+4; also avoids the problem. Apparently it's only  $n +4 that Perl looks at and says "Oh, of course: a unary plus!" (I haven't tested it, but I assume the same would happen | I've tested it, and the same thing happens with a pseudo-unary minus.)

Update: Per perlop, unary + and - have higher precedence than the binary operators.