As soon as they are assembled MMRTGs continuously produce about 2,000 watts of heat energy. That dips only slightly when they are connected to a load and producing electrical power (about 125 watts initially).
In deep space and on Mars some of the heat from curiosity's MMRTG was/is removed by circulating fluid to keep things from overheating during transit and to keep itself warm during the martian nights on the surface.
Space.com's Going nuclear: NASA's Perseverance Mars rover gets its power source for July 30 launch explains that Perseverance is now "hot" in more ways than one; it is now emitting roughly 2000 watts of heat continuously and has an (fairly) uninterruptible supply of electricity.
While the naked rover remains in an air conditioned laboratory this should be no problem due to convection at 1 bar, but it needs to be moved in a protected way and put into its "clamshell" for deep space flight and a payload fairing for launch.
(I've just asked What is the equivalent of Curiosity's "MSL" in the context of Perseverance? What's the official name of the mission? Is the distinction similar? because I don't know a better term than "clamshell" at the moment)
Question(s): From this point forward how is the heat being removed?
Presumably when in deep space the 2,000 watts is radiated fairly efficiently into space by some radiator exposed to "the cold of deep space" such that it doesn't have to get too hot to do so. But once in its "clamshell" how hot does its radiator have to be to radiate agains the walls of a room at 293 Kelvin rather than the cosmic microwave background of 2.7 Kelvin?
Curiosity's "clamshell" from this answer to Where does MSL end and Curiosity begin?
For more "clamshell" imagery see Why are planetary probe RTGs tilted at a jaunty angle?