Here's some random bits of Perl in my usual font:
And here's what that looks like with Fira Code:
Some of the changes are subtle, but I definitely like them. For example, with :: and .. and some other pairs, the two characters cuddle each other a bit more closely than usual, which helps with visual chunking.
The -> and => arrows are rendered as proper arrows, but my brain is slightly freaked out by these, after so many years seeing them rendered separately.
And then we have <=, >=, and !=, which look very different. The trouble with these is that my screen is effectively lying about what characters are in the file. Semantically the symbols are clearer, so I guess the idea is that your brain can visually process more effectively / efficiently, and it can handle the fact that what's onscreen isn't exactly what's in the file.
I forgot to include @ in the screenshots, but the @ in Fira Code is just weird.
There are a lot of other ligatures, many of them for specific languages. See the full set.
After a week the little voice in my head is still telling me to go back, but I'll try it for a bit longer.
I've switched back to Source Code Pro.
>= which did it for me.