Perot enabled Clinton to win a few states that he wouldn't have won in a heads-up race with Bush (e.g. Montana), but exit polling and postmortem research indicates that Clinton would have still won the Electoral College 281 to 257. Perot's 19% of the popular vote did keep Clinton below the 50% mark, but the exit polling reveals that Perot voters would have split roughly 50-50 in the popular vote (with a percentage not even voting) if Perot wasn't in the race.
Before Perot re-entered the race late in the summer of 1992, Clinton had a substantial lead over Bush in every tracking poll. After Perot re-entered, Clinton's numbers dropped, Perot's numbers went up, and Bush stayed relatively static. This shouldn't come as a surprise, because both Perot and Clinton were seen as candidates of "change", while Bush represented the status quo.
Perot's Impact on Clinton's 1992 Victory