well, Go has broken that backwards compatibility when it was deemed important enough to break things. So it's not "perfect" and I imagine Perl's isn't either.
Perl as a language is pretty mature and rarely sees breaking changes (and what breaking changes exist are in corners of the language that are extremely esoteric even for Perl). You can take a Perl script written 25 years ago and it'll still run just fine.
New language features and things that might alter existing behavior are usually locked behind a version declaration opt-in at the top of your script. You need to do something similar to `use v5.39;` to unlock new behaviors from that version of Perl -- which may contain breaking changes but those are also generally backward compatible too.
But that's for the base language and standard library. Individual third-party packages may vary in their backward compatibility.
I remember a breaking change that was not esoteric: "keys %hash" started producing a different result each time the script was run. It required replacing the construct with "sort keys %hash" to get reproducible results.
I can't remember a time where "keys %hash" returned the keys in a defined order. And I've been using Perl for over 30 years now.
I've just consulted my first issue of "Programming Perl" (printed in 1990l and it says:
keys (assoc ARRAY) this function returns a normal array consisting of all the keys of the named associative array. The keys are returned in an apparently random order, but it is the same order as either the values() or each() function produces (given that the associative array has not been modified)
> I can't remember a time where "keys %hash" returned the keys in a defined order.
It was never a _defined_ order, but before version 5.17.6 (November 2012), each hash returned its list of keys and values in a _consistent_ order between runtimes, so some code ended up getting written that depended on this ordering (say in a unit test, or a list that would get assigned into a database). The change was to make the ordering random/inconsistent/unpredictable every time the list was fetched, which as I recall did break some number of tests in CPAN modules and required new releases.
Agreed, but prior to the change the "apparently random" as described in the documentation was similar to using random(fixed_seed), rather than the new behavior of random(system_time), which made the output of the script non-reproducible given the same inputs. The change to Perl's behavior was made for security reasons.
