My guess is that for many people it's hard to write non-obfuscated Perl code. It has so many operators and ways of doing things that walking into someone else's code may feel like having to learn everything from scratch.
In all honesty I have read more unreadable code in Python, than Perl. And I have written nearly equal of both over my career.
You can write bad code in any language. Bad variable names(sometimes single alphabet names), functions running pages long, duplicate code, algorithmically inefficient code, no error handling, master try/catch statements, OO abuse, functions with unpredictable side effects etc etc.
Early internet saw a flood of newbie programmers, and therefore a flood of badly written code too.
For that matter you also see badly written C/C++ code from those days. How do you think C++ got its reputation for being too bloated beyond practical use?
(Higher Order Perl and such not withstanding.)