CAWless Mod is a modification that "cannibalizes" CAW Slots in order to add up to an additional 100 Wrestler Slots for menu-accessible mods to the game. The CAWless Mod has first been introduced as part of PacPlus included in WrestlePLUS for WWE 2k19.
Currently, WrestlePLUS' PacPlus is the only tool that can enable the CAWless Mod as it requires the change of the memory structure of the game which file-based tools like PerfectPlextra can't accomplish. WrestlePlus/PacPlus must be running at all times while you play the game in order for CAWless Mod slots to work - don't shut it off before exiting the game or your save game may get corrupted!
Using the CAWless Mod means you sacrifice and thus lose your CAW/CAS Slots in order to have more slots available for mods (note that this can lead to issues with the game, including loss of functionality that depends on CAWs as well as possible save game corruption that may be caused by this).