Perl 5 was released in 1994. Palo Alto Networks was founded in 2005, so Perl 5 was certainly available when they built their first products and would have been a superior choice to either PHP or Perl 4.
Sure, you can do web apps in Perl with Catalyst or other frameworks, but the PHP developers are easier to find.
And then, after you develop entire GUIs in a language, it is a herculean effort to port that to another.
And lastly, JavaScript has its own issues too.
It is a huge effort even migrating from one PHP framework to another.
Coming back to your point, there is no single “page” to be migrated one at a time.
Is there a reason one URL at a time wouldn't work?
You can always take a look at the UI of a PAN device and use your browser in developer mode to see how it works and understand the concept of a web application.
I do not have a PAN device.
> it is not "one URL"
I never implied it was.
> it is an entire application, it does not have distinct "pages"
> the concept of a web application
Those are usually written in javascript and the client part could stay the same. If anything that kind of design makes it easier to replace the backend one URL at a time becasue the responses from the backend are simpler.
If they have something where the UI state is kept on the server and is relayed to the browser, that would be difficult to do anything with. But that's an especially terrible design that causes other problems, so if you're accusing them of having a design that bad please be explicit.
Perl 5 was the language of choice for CGI in that era, but PHP borrowed some of perl syntax and quickly displaced Perl in web usage.
Some perl programmers migrated to Embperl, but it didn't gain much momentum.