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Do we know how fast Perseverance was flying at relative to Mars when it hit its atmosphere?
The closest figure I could get to was "over 12,000mph" according to this article: https://www.nytimes.com/live/2021/02/18/science/nasa-mars-landing#:~:text=At%203%3A48%20p.m.%20Eastern,than%2012%2C000%20miles%20per%20hour.
But it doesn't really cite a source and I'm hoping for a slightly more specific figure if possible.
Your number seems quite close to the one below, but that's not a surprise. There isn't a precise beginning to an atmosphere, so it doesn't make sense to look for an exact number.
Since the atmosphere varies exponentially, there is no edge to hit, but there is an entry interface, an arbitrary point where the mission planners simply say that the atmosphere begins for planning purposes.
Here's a bit of the moment-by-moment narration from Perseverance Entry, Descent and Landing Lead Engineer Dr. Swati Mohan who does an excellent job explaining what was happening throughout. Starting at about 03:05:40 in the video WATCH LIVE: NASA's Perseverance rover landing on Mars
We’re just under two minutes from entry interface. As it gets closer to Mars, Perseverance gets pulled in by gravity and accelerates. By the time Perseverance reaches entry interface point, it should be going just under 5.4 kilometers per second.
That's about 19,400 kph or 12,100 mph.
Screen shot showing the atmosphere entry countdown clock to the somewhat arbitrarily defined entry interface point:
