|Perl Monk, Perl Meditation|
> AFAIU this calls a js function checkform() prior to submitting.
It's a classic JS form validator, a submission is only possible if it returns true.
> For example, look at the B1=Submit field. Where does that come from?
I can see this with JS disabled
<input style="font-size: 13pt" type="submit" value="Submit" name="B1">
I don't have the impression JS is adding something to the form, frankly I doubt the author had the necessary skills to do so.
But my advise here can only be superficial, because I don't know what the correct genetic input has to loo like. And the author decided to let his web-service simply crash, when the JS validation is bipassed to send broke input. 🤷🏽
DISCLAIMER: I don't expect biologist to be good hackers, but academics should be capable to ask proper questions.
ANYWAY: I agree with you that this is not really a Perl issue, it's about HTML/HTTP.