The only way outside of disabling the warning is to add extra parentheses so that Perl knows that you meant to write the code that way:

which( ( $a < 1 ) | ( $a > 3 ) );

Am I missing something? I see that warning as a good thing, and indeed that one should use parens. If one wants to suppress it, no warnings is how. @stphnlyd Thoughts?

Thanks @zmughal, @mohawk2 ! I am new to PDL, and I've not been using bitwise operator in Perl for long that the parentheses thing did not come to my mind. And you know R's which() method does not require parens in this case. I think this ticket can be closed now, but I wonder if it could be better to add an example to which() in PDL::Primitive, so that people like me can just figure it out.

@stphnlyd You're probably right about the example. PRs welcome :-) I'll close this issue.