From the Perl 5.30 documentation:
> WARNING: Subroutine signatures are experimental. The feature may be modified or removed in future versions of Perl.
> WARNING: Subroutine signatures are experimental. The feature may be modified or removed in future versions of Perl.
Which is to say: a lot of the reason they're still experimental is because Perl's commitment to backward compatibility. Other languages might have said "oh well," but Perl 5 said "oh, an unintended consequence that might break real uses in the wild; we should wait another year."