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The Amazon retail site is written in perl (at least the presentation layer still is, you can find mason references still in the HTML source if you go looking, don't know how much else behind it is in perl).

https://metacpan.org/pod/Mason (if you're not familiar with Mason)

Actually, that's Mason 2. Amazon use(s|d) the OG Mason - https://metacpan.org/release/HTML-Mason

Duck Duck Go and Booking.com were/are Perl backends.

The fashion retailer Net-A-Porter has been a Perl shop since 2000. Haven't checked if that's still the case. They used to employ quite a few leading lights in the UK Perl community a few years ago.

I think they still write everything in Perl, and they employ Larry Wall (or they did several years ago, last I heard)

I know some fantasy sports site was, maybe CBS? I had a UX designer colleague who had worked on that.

A surprisingly large number of fantasy sports sites are.

Perl is everywhere, it's just not widely publicised because of the stigma attached to the language.

duckduckgo, as far as I recall.

imdb.com is perl.

Most of the ad network backend sites are perl, including a lot of Yahoo.