Prejudice means literally, "pre-judge". As in judgements that you make prior to having sufficient information to make a good judgement.
There are two major consequences of this. First of all, since you are judging on highly incomplete information, your prejudices are guaranteed to sometimes lead you to make wrong decisions. Secondly, since we are often in a position where we need to make decisions based on partial information, we need at least some prejudices.
So the question is whether it is a worthwhile prejudice to make a snap judgement about people and companies that say PERL. It is definitely a prejudice. It is definitely sometimes wrong. But is it worthwhile to make this facile judgement?
My personal belief is that in most situations it is worthwhile. Let me invent some sample figures and run some numbers to justify it.
Sturgeon's law says that 90% of everything is crap. He was probably an optimist. But let's say that he's right. Let's say that 3/4 of bad Perl programmers (and jobs) say PERL, while 3/4 of the decent ones say Perl. Then over 96% of people who say PERL are bad, while 1/4 of people who say Perl are decent. Given that 70% of people say PERL, this rule really improves your chances of finding a decent programmer (or job, depending on how you use this) after a relatively short search. Yes, you'll make mistakes. But this isn't proving to be a bad initial filter.
Now if I'm really motivated, I'm going to plumb that remaining 70% for that the 3.6% or so of decent possibilities in the pot that I'll otherwise miss. But in real life I'm rarely so motivated.
Yes, I know that means that I'll miss good people. Yes, I know that it is horribly unfair to those I'll overlook. But life isn't about being perfect, it is about making reasonable trade-offs. In fact trying to be perfect is itself an unreasonable trade-off - you wind up putting too much work out for too little reward.
So your anecdote notwithstanding, I'll continue to apply this prejudice in all situations other than ones where I think that the effort of an exhaustive search is worthwhile.