That is a linguistic phenomenon called "apocope" or "troncamento". I found an explanation (in italian) of how it works here: http://forum.wordreference.com/showthread.php?t=618723 I hope that will be useful. I also apologize for my poor English.

Hi Yan, this is a simple licence to make more speedy and fluent the sentence that contains these verbs shortened, in fact they would be considered as errors, but it's common use to "eat" the final vowel in some verb. You can find something similar also in american slang: "d'yo think ...; I gotcha ...; I don't ... (do not); I've ... (I have); etc. Ciao

Sometimes you can meet other similar phenomenon like "col" instead "con il" or "colla" instead "con la" (with the) usually when there's apostrophe. Actually these forms are used mainly but not only in the spoken language. You will hear sentences like this: "Io sono tornato [io tornai] a casa coll'autobus" [con l' ] (I came back home by bus) "Maria e' uscita colle [con le] amiche" (Maria went out with her friends)

I think It's the same linguistic phenomenon of callin' instead calling or jumpin' for jumping. (for having said, for havin' said) "per avere detto" or "per aver detto" are the same.

sorry "similar phenomena" :)