Document appearance of "defer" feature in perlsyn.pod

[dboehmer]

Perl version that introduced defer taken from #17949 (comment)

[leonerd]

Locally, this change appears to be fine. But on a wider perspective, we don't currently document when various syntax features first appeared. If we add it here is raises the question of whether many more of the recent additions should be similarly documented.

I think they probably should, but before we merge this one in particular we should decide on a wider view, if that's the sort of thing we want to do.

[dboehmer]

I actually tried to find out when defer became available and didn’t find a list. perldoc feature does also only mention that it is experimental.

Both perldoc feature and perldoc -f defer refer to perldoc perlsyn. So I looked there and several syntax features are already described with the Perl version required:

• when https://github.com/Perl/perl5/blob/07a53dca6ee0b9c82acfb8d777fe4e7cc6aae661/pod/perlsyn.pod#219
• for lists with multi-value assignments https://github.com/Perl/perl5/blob/07a53dca6ee0b9c82acfb8d777fe4e7cc6aae661/pod/perlsyn.pod#285
• yadda-yadda operator ... https://github.com/Perl/perl5/blob/07a53dca6ee0b9c82acfb8d777fe4e7cc6aae661/pod/perlsyn.pod#995

[khwilliamson]

Other commits that add similar information have been approved in recent releases, in spite of such concerns.

I think the de-facto rule is "If this bothered someone enough that they created a PR, then accept it".

I think that is reasonable, so this LGTM

[leonerd]

Fair enough. In that case we'll merge this one now and we can make a more thorough sweep of perlsyn, etc.. to add similar notes about when other recent additions appeared separately.