Front-runner Donald Trump swept all of Tuesday's five GOP presidential primaries by large margins. In a speech after the victories, Trump said he considers himself the "presumptive nominee."
"Just because you're leading in the fourth quarter," the game is not over, Perry told CNBC's "Squawk Box."
The former Texas governor said while he supports fellow Texan Cruz, he would stand behind whoever emerges as his party's nominee.
With the possibility that Trump could be short on the 1,237 delegates needed to clinch the nomination before this summer's convention, the New York tycoon has repeatedly complained about the rules surrounding the primary process, arguing he's by far won the most votes.
If Trump doesn't like the rules, he shouldn't have run as a Republican, Perry said. He likened Trump's grievances to Democrat Al Gore complaining about not winning the 2000 presidential election.
"I just know how the rules are set up. And if you're not going to follow the rules, why are we a rule-of-law country?" Perry asked. He said a delegate count under 1,237 opens the door to a contested convention.