When in doubt, Deparse. In this case we want extra -parentheses.

$ perl -MO=Deparse,-p -e'if(not defined $username || $username eq ""){
+}'
if ((not (defined($username) || ($username eq '')))) {
    ();
}
-e syntax OK
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Apparently noone's gut feeling was entirely correct - mine neither. Don't try guessing.

Makeshifts last the longest.