Presumably it is a line-item veto abuse, see here and here. (per the UPDATE links it was a line item veto exercise by Perry, to coerce/induce a political officeholder change along party lines, and he said as much before the veto action).
Even if the indictment does not stick, rolling down a public integrity unit absent a power to put a crony in the driver's seat is very shabby, and reflective of who Perry is, a political bully. And an ass. In that order.
_________UPDATE__________
If the bullying of the county prosecutor in the Austin area also involved a side offer to the prosecutor, was it bribery? Erica Grieder writes, here and here. Yes, it was a grand jury - a citizen panel - that indicted. But with grand juries there is only one lawyer in the room, and that leads to frequent indictments in flimsy situations. It is not adversarial, hence it is not two sides of an argument presented to the citizen panel. Go figure.