Back in June it was announced that the next Perl release would be called Perl 7. A quick look back at Perl's history reveals that this is a little unusual. Perl 5 was originally released in October 17, 1994 and served its users well for a number of years. In 2000 Perl 6 was announced as a ground-up redesign of the language and it spent a long time in development. It eventually surfaced in 2015 but in 2019 was renamed "Raku" to distinguish it from traditional Perl. This leaves Perl 6 as a sort of "orphaned" version number.

In the meantime while Raku née Perl 6 was being developed, the team behind Perl 5 had delivery numerous point releases and retained an active community of loyal users. Many of these had no desire to migrate to Raku and wanted a number of quality of life improvements in their language, ecosystem and libraries. The sort of changes being proposed were bigger than you'd expect from a point release, so a new major version was required. Hence Perl 7 - the real next version of Perl.

So we now have quite an interesting situation, let's take a look at the release history:

Over the years many commercial and open source projects have skipped version numbers leaving their own little version holes with their own various explanations. Today we'll take a look at a number of similar situations like this, and we'll try to understand what happened in each case and why.

A fairly typical Perl program which calculates π

PHP 6

Perl was not the first language to skip a version, in fact it wasn't even the first scripting language beginning with "P" to skip version 6. A quick glance at PHP's release history reveals the following:

So what's the story here? Well in common with most software written in the early '00s PHP did not have particularly good Unicode support. The developers behind PHP recognised this and planned a rewrite - PHP 6 - that used Unicode to represent strings internally, specifically UTF-16. Work began in 2005 and would continue for a few years. While some of the planned PHP 6 features were completed on schedule, the Unicode rewrite stalled and became a major blocker to the release. In the meantime Unicode did indeed begin to take off ...

... however UTF-16 didn't. UTF-8, a different standard, rapidly emerged as the preferred Unicode standard on the web. This was particularly problematic since it became apparent that PHP 6 had some serious performance issues dealing with converting strings to and from UTF-8, and since web applications were PHP's bread and butter this was a big deal. In 2010, five years into the PHP 6's development the project was abandoned. Much of the non-Unicode features of PHP6 were ported into PHP 5 and released in version 5.4.

In 2014 as a new major release of PHP was being planned the topic of what it should be called was raised and it was decided that for the sake of removing any confusion or ambiguity the next version of PHP would be PHP 7. There are plenty of justifications given, but the first one given in the RFC I linked really seals it

First and foremost, PHP 6 already existed and it was something completely different. The decimal system (or more accurately the infinite supply of numbers we have) makes it easy for us to skip a version, with plenty more left for future versions to come.

This makes plenty of sense - PHP 6 was already a thing, and it's not like integers are a finite resource. If this was Ubuntu and they were considering discarding a good adjective/animal pair then perhaps it'd be slightly different

ECMAScript 4

Brendan Eich developed Javascript over the course of 10 short days at Netscape, and it soon became the defacto scripting language for the web. This status was confirmed by Ecma who issued a number of standards and referred to it as "ECMAScript". Nowadays they release these standards on a yearly basis (the latest being ECMAScript 2020, published June 2020) but at the end of 2009 we had five ... kinda:

You seem to be repeating falsehoods in blogs since the Proposed ECMAScript 4th Edition Language Overview was published, claiming dissenters including Microsoft were ignored by me, or “shouted down” by the majority, in the ECMAScript standardization group
...
These ES3.1 proposals came suddenly in March after years of passive participation by Microsoft, while the rest of the group worked on ES4. They have not been touched since April.

Watcom 5.0 and earlier

Slackware 5 and 6

For some reason Slackware was the first ever Linux distro I used. I have no idea why I chose it, but looking back it would've been better if I had a more user-friendly one like RedHat or SuSE. Using software on Slackware often involved compiling it yourself, which usually meant hunting down, downloading and compiling dependencies (possibly even encountering more dependencies which require you to hunt down, download and compile sub-dependencies (possibly even encountering more dependencies ... etc)). Between May and October in 1999 they jumped from using 4.0 to 7.0

Conclusion

I only scratched the surface of this subject, really. If look around then you'll start to see missing versions everywhere. For example, what happened to Windows 9? How about .NET [Core] 4, MySQL 6 and 7 or Akumulátor 2? As soon as you apply an sequential version to some software you make a statement about how that software relates to any previous or future releases - whether or not you adhere to a strict set of rules like semver. This is meant to help users and it largely works as intended but does not really give any strong prescription for how to handle abandoned releases or forked-then-remerged projects. As a consequence we have to be cool with the idea that skipping a version or two is perhaps preferable than introducing confusion or even discarding any notion of sequence whatsoever - imagine wondering if it's safe to upgrade from PHP e1daa19 to PHP caf3c64.