I don't know all the details
In a few words,
given, lexical $_ and smartmatching were added to Perl without enough vetting in 5.10.
In fact, smartmatching was so bad that it was tweaked in 5.10.1.
People starting noticing the problems with them, which resulted into a new policy: New features to be added to Perl would first be included as experimental features. By default, these warn when used.
given, lexical $_ and smartmatching were retroactively converted into experimental features in 5.18, effectively deprecating existing uses of them.
Lexical $_ was rolled back in 5.24.
given and smartmatching are being revamped for 5.28. It will still be an experimental feature, but it now has a chance of being released as normal part of Perl.