All arithmetics in Perl is performed in binary. Though that's not really relevant, because the outcome would be the same if it were performed in any other base.

$ perl -wE 'say 4 + 5'  # uses binary addition under the hood
9
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It's just the input and output of numbers that decimal by default; but as others have shown you, there are ways to achieve binary input too.

So I guess the real question is: what do you want to achieve?