It was always easy to write unmaintainable code in. As time went by other languages caught up with the things Perl did well (dependency management with a large base of libraries available, automated tests that you could have confidence in because they ran by default as part of installing any package, lightweight syntax that makes easy things easy, easy to make available in a shared hosting environment), whereas Perl's biggest problem was inconsistency which was essentially impossible to fix (except with a big rewrite of the whole language, which they did try, but...).