Assume a cleric has converted one of their first level spells into cure light wounds, and it was the only spell they have expended. When they use the pearl of power to recover the spell, what is recovered: the original spell, or the cure light wounds?
The cleric recovers the original spell…
On Spontaneous Casting says
The cleric can 'lose' any prepared spell that is not an orison or domain spell in order to cast any cure spell of the same spell level or lower (a cure spell is any spell with 'cure' in its name).
The cure spell that was spontaneously cast wasn't prepared. The original spell was. The description of the pearl of power, in part, says
Once per day on command, a pearl of power enables the possessor to recall any one spell that she had prepared and then cast that day. The spell is then prepared again, just as if it had not been cast.
Thus using the pearl restores the original spell… that the cleric can then expend again to spontaneously cast a cure spell, if she so desires.
…Or, strictly, neither spell
A comment by Ilmari Karonen pointed out a quirk in the pearl's description. A GM could read part of the pearl's description—that it "enables the possessor to recall any one spell that she had prepared and then cast that day" (emphasis mine)—as it forbidding the caster from using the pearl to recall a spell that was "lost" due to spontaneous casting.
This GM and player doesn't endorse such a reading, but in, for example, a campaign dominated by casters with the ability to spontaneously cast spells, this GM could imagine another GM going with such a literal reading.