You would be better off asking us how to address the security concern that you are being opposed with than you are asking us to find a security problem with the old version of Perl. It may be that the issue is a non-issue. It may be a real issue but someone is saying "security" when they really mean something closer to "control". It may be that you would be violating a security policy which exists for a good reason, and that point is a blow against you no matter how you cut things.

I would be more specific on that, but I have no idea what security objections you are being confronted with.

Instead I will be specific with what you said here. You asked about security problems with a specific version of Perl, and then said that you wanted to know this to try to get them to upgrade. That is clearly a case of a conclusion begging a line of reasoning. My experience is that people who rightly or wrongly give that impression quickly find any reasoning that they present promptly dismissed by everyone who knows them because of the obvious bias.

Indeed I would tell you that in the long run (and the run need not be very long), being seen as willing to lose a battle because the immediate facts don't support your side will win you more battles fairly shortly thereafter. Sometimes you even wind up winning the original battle that you gave up. If there is a security objection, and the objection is fair, a willingness to acknowledge that and work with the objectors to address rather than dismiss their objection may be the best way to get results.