Corbett spokeswoman Janet Kelley said Corbett had not seen the letter and had no immediate comment.
The moratorium request came amid efforts by death-penalty opponents to stop the Oct. 3 execution of Terrance "Terry" Williams, 46, for the 1984 murder of Amos Norwood, 56, in the city's West Oak Lane section.
Williams was privately interviewed Thursday by the state Pardons Board on his request for a recommendation of clemency asking the governor to commute his sentence to life in prison without parole.
The board will hold a public hearing Monday in Harrisburg. The five-member board must unanimously vote for clemency in order for Corbett to consider the nonbinding recommendation.
Corbett signed Williams' death warrant Aug. 9 in what would be the state's first execution since 1999 and its first uncontested execution since 1962.
Williams' lawyers have cited his youth at the time of the killing - he was just three months past his 18th birthday, the minimum age for execution in the United States - and the fact that Williams had been sexually abused by Norwood for several years.
Williams' lawyers say more than 25,000 people have signed in support of his clemency petition. Last week, Philadelphia Roman Catholic Archbishop Charles J. Chaput called for clemency, followed this week by the Pennsylvania Catholic Conference, the church's lobbying arm in Harrisburg.
The task force letter, however, said the moratorium was not related to Williams' pending execution. Rather, the letter reads, "carrying out an execution before our work is completed over the next 15 months would greatly undermine . . . a comprehensive study of the effectiveness of capital punishment in Pennsylvania as it pertains to cost, fairness, proportionality, impact, and many other factors."
According to state prison officials, Pennsylvania has 200 people - 197 men and 3 women - awaiting execution at five state prisons.
Contact Joseph A. Slobodzian at 215-854-2985, firstname.lastname@example.org, or follow @joeslobo on Twitter.