Can I use an L7805CV with an AC input?
No buddy , you can not connect 12VAC directly to 7805 as it is not meant to be operated with ac voltage , 
better use bridge rectifier and capacitor afterwards to smoothen the DC output and then feed it to the 7805 to get +5VDC.

12VAC * 1.414 = ~17VDC 

With this 17VDC input and 5VDC output - if your current drawn is high , definitely your 7805 will get hot so you may need a small heatsink to dissipate the heat and keep the junction temp. of 7805 less than absolute maximum .

hope this helped !