From the Lua Reference Manual:

Enters an interactive mode with the user, running each string that the user enters. Using simple commands and other debug facilities, the user can inspect global and local variables, change their values, evaluate expressions, and so on. A line containing only the word cont finishes this function, so that the caller continues its execution. Note that commands for debug.debug are not lexically nested within any function, and so have no direct access to local variables.

function love.keypressed(key, u)
   if key == "rctrl" then --set to whatever key you want to use

Caveat: The debug mode can only be activated if there is a terminal bound to LÖVE. On Windows, launching LÖVE from the command prompt is not sufficient. Instead, run LÖVE in console mode with the command line argument "--console" or the configuration file option "t.console = true".