Back in the early days of the web (I'm talking 1993-1995), perl had a bunch of huge advantages:
1) It derived a lot of syntax from the classic unix tools. So if you knew sh, sed, awk, troff, grep, you were halfway to knowing perl.
2) The performance of fork() on many unixes was bad. Or at least, bad enough to make you avoid starting a lot of processes. With perl, you could usually do everything in-process.
3) Bash existed, but did not have the wide acceptance that it has today. Remember, Linux was brand spankin' new, and we were mostly using proprietary SunOS/Solaris/HPUX/AIX systems at the time. Those systems which shipped with /bin/sh, not /bin/bash. Heck, most sysadmins did not bother to install bash, because what as the point of installing some non-standard buggy and redundant shell? (Whether or not if this was true at the time is more up for debate, but that was the attitude.)
4) Bash didn't have associative arrays until much later, I believe.
5) There were things awk just had trouble doing, or at least doing quickly. Perl had a lot of the sysadmin-y system calls available as pure perl functions.
6) Perl's regular expressions were second-to-none
7) The comp.lang.perl community was probably the best, or at least busiest, of the applicable newsgroups at the time.
8) The perl community was very quick to provide example CGI code, so if you wanted to write a CGI script, you could bootstrap yourself very quickly with perl.
If you can ever find a copy of the 1991 edition of Programming Perl, you should pick it up--it was an amazing introduction to the language, and explains a lot of the advantages over other scriptish languages at the time.
And of course when Perl 5 came out, the language gained another huge advantage with the module system and pseudo-OO features.