Alien, taking a screenshot of a given HTML page, implies that the mentioned HTML page has been rendered, which is a quite complex task, handled with many difficulties even by modern web browsers.

Your best option is thus probably using a browser (on the server side by your Perl script) to fetch the given URL and to render the corresponding page, and then using some program to take the actual screenshot (such as scrot), as already suggested also by others.
It's likely that the PHP script you saw used some external HTML rendering engine/widget under the hood.

Ciao,
Emanuele.

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