To do anything like a "fiend" means to do this like an addict.  Pony means that Pepsi is like a drug to which he is addicted, and he drinks it like an addict would use cocaine.  Because Pepsi has both lots of sugar and caffeine in it, the drink would keep him hyped up with lots of excess energy.  Sometimes when people are too hyped up, they don't think first before they act and later regret it.  Someone with lots of energy can also be fun to be with unless the person lets the energy get out of control.  The connotation of "fiend" is slightly negative when used in this way.