While Mr. Perot did drop out in July, he re-entered the race in October. (http://www.nytimes.com/1992/10/02/opinion/the-uses-of-ross-perot.html) The results at the election were better than most third-party attempts (15% of the national vote), but ultimately not especially good for a three-way race. Polling done before the election was mostly in line with that.

Perot's campaign at the time before his withdrawal was falling apart. While he enjoyed an early polling success, the numbers were falling quickly and his staff, such as Ed Rollins, were departing due to conflicts in the campaign. That, compounded with gaffes and other missteps killed his chances. (http://www.nytimes.com/1992/07/16/news/campaign-ross-perot-rollins-quits-perot-s-campaign-asserts-his-advice-was.html)

If his campaign was better run and had fewer distractions, that would be a different story.