Anti-war activists continue to defy a ban on protests
BBC defence correspondent Jonathan Marcus who is covering the conference in Munich says there are growing fears among America's allies that more hawkish voices are in the ascendant in Washington.
Addressing the annual security conference, which brings together key officials and strategic experts, Mr Perle said the US had "never been more willing if necessary, to act alone".
"One hopes that won't be necessary - but I can promise you that if we have to choose between protecting ourselves against terrorism or a long list of friends and allies, we will protect ourselves against terrorism," he said.
Earlier, US Deputy Defence Secretary Paul Wolfowitz - regarded as one of the most hawkish members of the administration - said there could be no single all-embracing coalition.
He said what was needed were different alliances suited to different missions. Some allies might join with the US publicly, while others might choose quieter, more discreet forms of co-operation.
He added that Nato's role was still important, but stressed that the western defence alliance needed new capabilities to face new challenges in a new era.
Mr Wolfowitz said that since the 11 September suicide attacks on the US, "we have acquired a visceral understanding of what terrorists can do with commercial aircraft".
He added: "We cannot afford to wait until we have acquired a visceral understanding of what terrorists can do with weapons of mass destruction."
Tight security
About 400 defence experts from almost all the Nato countries are attending the conference to discuss the fight against terrorism and the alliance's planned expansion.
Police dispersed several thousand anti-war protesters who took to the streets of Munich on Friday evening despite a ban on demonstrations during the two-day conference. Several dozen were detained.
The authorities are hoping that checkpoints in the city, on roads leading into it and at the railway station will prevent trouble.