$ perl -wle 'my $param = "foo"; if ($param != null) {}'
Unquoted string "null" may clash with future reserved word at -e line 1.
Argument "null" isn't numeric in numeric ne (!=) at -e line 1.
Argument "foo" isn't numeric in numeric ne (!=) at -e line 1.

##</code><code>##

$ perl -Mstrict -wle 'my $param = "foo"; if ($param != null) {}'
Bareword "null" not allowed while "strict subs" in use at -e line 1.
Execution of -e aborted due to compilation errors.