Even then, I bet avoiding magic is still faster.

my $byte = u8( $mmap, 0x12346789abcd );
++$byte if cond;
u8( $mmap, 0x12346789abcd, $byte * 3 );
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If you search PM, you'll find benchmarks between 4-arg and l-lvalue substr. I don't remember how much slower the latter is, but it's quite a bit slower.