The couple, which split for the second time earlier this year (13), was spotted chatting and flirting with each other at the bash.
A source tells Us Weekly magazine, "They seem to be back together-ish. They were very flirty and having a really good time together. They have good chemistry."
Other party-goers included Mindy Kaling, BJ Novak and Beth Behrs, as well as Perry's actress pal Allison Williams and her boyfriend Ricky Van Veen, who is one of Mayer's closest friends.
Audiences may be divided over whether this comedy crosses the line as it looks for...