I just read about onomatopoeia in the “Dictionary of Basic Japanese Grammar”.
Page 50, it says that onomatopoeia are considered as quotes, and are marked as such by the “to” particle.
It is quite an insighful section of the dictionary, actually. In Japanese, onomatopia are not fully random words, or childish words, but there are fully part of the language. The phonemes in the onomatopia gives out some information about its meaning and the state of mind of the speaker.
Here the らら pattern relates to the notion of fluidity, smoothness.
And ペ* relates to explosiveness, strength.
ぺらぺら is part of the example illustrating strengh-related onomatopia.
This grammar dictionary is a bit dry, but I am planning to continue reading it from cover to back. The fact it only contains “basic Japanese grammar” means that I would certainly need such level of details at some point.