If by “hard to understand” you’re including trying to understand someone else’s code, Perl can be a nightmare.

I’ve heard a few people refer to perl as a “write-only” language.

There’s some truth to it, but it’s mostly that junior developers and senior developers with no discipline that give it a bad name.

The major problem is that it has one of the the highest capacities for writting incredibly dense code there is, paired with very powerful language transformation tooling (i.e. switch statements were added as a module, but can also be used for funny evil.)