European champions Milan, who have a game in hand on their title rivals, are level on 36 points with second-place Juventus, who beat Sampdoria 2-1 in Genoa thanks to a Mauro Camoranesi header and an Antonio Conte strike.
Lazio suffered another setback with a 1-0 home defeat to Brescia -- former Italy international Luigi Di Biagio scoring the winner.
On Saturday, fourth-placed Inter Milan lost 1-0 to financially troubled Parma, who are fifth two points ahead of Lazio.
Milan suffered an early shock when Reggina defender Stefano Torrisi put the visitors ahead in the first minute, but Kaka put Milan on level terms just seven minutes later with a superbly-taken individual goal.
The midfielder's second, which put Milan in front in the 55th minute, was a more fortunate effort -- an attempted clearance from Reggina defender Andrea Sottil hitting Kaka and flying past goalkeeper Emanuele Belardi.
The 21-year-old Kaka, who has made a big impression since arriving from Sao Paolo in the close season, had a major hand in Milan's third when he was brought down by Martin Jiranek and Andrea Pirlo converted the penalty with a cheeky chip from the spot.
NEDVED DECISIVE
Sampdoria went into their game with Juve unbeaten in seven games but once again European Player of the Year Pavel
Nedved created Juve's opener in the 23rd minute, winning possession and bursting out to the left flank from where he fired over a pinpoint cross which was met by a diving header from Camoranesi.
Sampdoria could face disciplinary action for an incident after the break when Juve's Italy international keeper Gianluigi Buffon was struck by an object thrown from the stands.
The home side continued to put Juve under pressure and drew level through a Francesco Flachi header from a Sergio Volpi cross 11 minutes into the second half.
But Juve secured an important win in the 64th minute when Conte blasted home after a delightful back-heeled set-up from Nedved.
Roma, whose unbeaten run was ended by Milan last week, still looked short of their best at winless Perugia but will settle for the three points gained after Mancini collected and converted a pass from French midfielder Olivier Dacourt in the third minute.
Their city rivals Lazio are all but out of the title race, now eleven points behind the leaders after a second successive defeat to lowly opposition following last week's loss to Reggina.
Lazio, with Claudio Lopez their only fit striker, struggled against Brescia after falling behind to Di Biagio's fifth minute indirect free-kick, which took a nasty kick as it bounced past Angelo Peruzzi.
Stefano Fiore hit the post for Lazio but Brescia's Roberto Baggio could have even added a second near the end.