It all went down at Perla's L.A. home on Saturday night -- where everyone was encouraged to dress up as their favorite divorcee ... and the guests came through strong!

Perla dressed up like Zsa Zsa Gabor. One guest dressed like Gwen Stefani. Perla and Slash's son, London, went as Tommy Lee, complete with the stomach tattoo.

London and his band Classless Act even performed at the rager -- with Perla making a cameo on drums.

But, the diss-de-resistance was the cake -- which showed Perla standing on top of a smashed out Slash with the message, "Happy New Beginnings Perla."

The two were married back in 2001 and split in 2014 -- but the divorce just became finalized last month.

Despite the uncoupling, the bad blood is still very real -- in fact, Slash has told Perla she's persona non grata at the Hollywood Palladium Tuesday night, where London and his band are opening up for Slash.

Perla has said she plans to attend the show anyway -- which means Slash has a real decision to make.