I'll double check, but I'm pretty sure Perot was over 20% at some point PRIOR to the debates (and prior to originally dropping out). Even if they did institute the 15% rule after him, which I'm not sure on that but is possible, he'd still have met that requirement.
Ah, here we go...
Gallup Poll. From April to Mid June, Perot managed to poll at 24% or higher, topping out at 39% (actually leading Clinton and Bush). Mind you this was before any "debates" occured and he managed those numbers. While his numbers prior to the debate...and note, post him dropping out of the race...had dropped, starting at 8% when he came back in and hovering in the low 10's prior to the debate (The Gallup picture doesn't show specific dates). It did rise some, but even after getting on the debates Perot never managed to get back to the numbers he originally was showing. But its unquestionable that during the year of the Presidential election Perot managed to get above 15% in a national opinion poll. Not only did he get above it, he managed to more than double that amount.
Gallop was the only national poll I could easily find information on for back then.
Conversely...
Johnson has managed 7% support in a Public Policy Polling national poll recently. Beyond that he's not been listed, but "other" has managed 1% in a CNN naitonal poll, 1% in an ABC/WaPo national poll, and 3% in a Fox News national poll. He doesn't seem to blip on the gallop radar on first look.
Perot, prior to debates, managed to make a compelling enough appeal to the American people to garner significant and substantial public support to the point where he actually lead in a national poll during the election year. He gave compelling and significant reason why he should be part of the debates...not because he needed them to get support but because he HAD support and thus should be in them. Johnson can't even crack 10% support at this point in one poll let alone potentially consistently over a few.