Given that Sys::Mmap ties a scalar to the memory region returned by OS-level mmap() and that you get exponential run-time (and not just a constant increase) when using Sys::Mmap, my guess is that the regex seeks at the beginning of the string for each iteration of the loop but somehow/possibly does not mess results up.

If you avoid using the tie'd variable, you should get equal performance. E.g., my $str1 = $str; and apply regex to $str1.

Each time Sys::Mmap's tied scalar FETCH()es, it dereferences the original scalar ref (see https://metacpan.org/source/SWALTERS/Sys-Mmap-0.19/Mmap.pm). Perhaps the dereference of the mmap-ed string combined with regex-ing it in a loop via the g modifier causes the problem.

bw, bliako