It makes little difference for Perl that this experiment in a next generation version of the language is still alive and going its own way. The result is the same...people kept moving forward with version 5, and if it needs a new version, it can't use version 6 because 6 was already used for that other experiment (that failed to take the place of Perl 5).
The primary goal of PHP6 was to implement full unicode support and drop mbstring. However, that took much longer than expected and multiple major features ended up getting backported to 5.3, 5.4 and 5.6; to the point that PHP 7 became the new feature version while PHP6 was worked on. They eventually gave up and PHP7 was released with unicode support built fairly deep, but basic string types and the like still being byte arrays.
PHP7 was PHP6, without the native and full unicode coverage requirement.
"after DirectX 3 was released, Microsoft began developing versions 4 and 5 at the same time. Version 4 was to be a shorter-term release with small features, whereas version 5 would be a more substantial release. The lack of interest from game developers in the features stated for DirectX 4 resulted in it being shelved, and the corpus of documents that already distinguished the two new versions resulted in Microsoft choosing to not re-use version 4 to describe features intended for version 5."
It seems Unix vendors really don't like low version numbers. Of course, Windows famously jumped from 3.x to 95, so it's not like they're any better.
Microsoft was surprisingly good at not skipping numbers, until the jump from Windows 8.1 to Windows 10, which also lead to the NT kernel jumping from 6.x to 10.
A lot of programs refused to work if there was a `9` in the version name.
That was of course because those programs needed something newer than Windows 95 or Windows 98. So they just did a string search for `9` in the version name.
There may be a few programs that wouldn't work on `8.9` or `10.9` if they were released for the same reason.