I was half right. The warning comes from op.c, specifically Perl_scalarvoid. When it hits a constant op, it checks to see if it should throw a bareword warning. Otherwise, if it needs to warn about useless constants in void context, it does. There's a hack, though, like I thought, but the comment says:

/* the constants 0 and 1 are permitted as they are
   conventionally used as dummies in constructs like
   1 while some_condition_with_side_effects;  */
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