$ perl -wle " print +foo"
Unquoted string "foo" may clash with future reserved word at -e line 1.
Name "main::foo" used only once: possible typo at -e line 1.
print() on unopened filehandle foo at -e line 1.

$ perl -wle " print -+foo"
Unquoted string "foo" may clash with future reserved word at -e line 1.
-foo

$ perl -wle " print -'+foo'"
-foo

$ perl -wle " print -(-foo)"
+foo