From a numbers perspective, it does seem that Bush may have won if Perot did not run. However, Perot was also a more moderate conservative than Bush and had a populist anti-NAFTA appeal that probably attracted some Democrats.
This is certainly a common line of thinking in some circles, but the numbers don't really suggest that was the case.
Immediately after Perot bowed out of the race, Clinton gained 13pts in the polls while Bush gained 15, a combined gain of 28pts (Perot had polled at 33 before leaving). However, Bush's support evaporated by August, and never really came back, even in the dead heat.
This seems to suggest that Perot's supporters split relatively evenly between the two candidates, not to mention that some would undoubtedly sit out the election were it not for Perot. The '92 race had the highest voter turnout in decades, in no small part because of the novelty of a viable third party candidate. Just because they were compelled to the polls by Perot's charisma, however, did not mean they automatically would've gone for one of the other parties if he dropped out.
Finally, exit polling suggests that Perot's remaining support split 38-38 between Bush and Clinton, had Perot continued to be off the ballot. Even if Bush had taken the remaining 24% of those voters (who are more likely to have just sat it out), Clinton would still have reached 50.2% of the vote, which would still give him a very reasonable shot at winning, even in the worst case scenario.
Perot would be rated as a moderate liberal today by Ontheissues.org