Some modules have a built-in solution for working without XS. The most famous example is Math::BigInt works without XS, but has XS parts separated to three other distributions you can install and then you can ask the module to do the same things faster. A few other similar modules are Text::Levenshtein and Heap::Simple. Another module group, Digest, has some hashing methods implemented in pure perl, others in XS, so you can still use e.g. CRC hashing if you don't want to install XS. I don't exactly know how List::Util and Scalar::Util handle pure perl installations, but both definitely install as an XS module normally and have pure-perl code for most functions as well.