For backwards compatibility with program/modules that have a sub/method called say.
You do not need to enable a feature to use the smart match operator. You do for given and when, though, for the same reason as say.
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You're kidding right? Why can't those program modules what have a sub/method called say simply rename it? I doubt that every new function that has been introduced worried about possible name conclusions of functions in the past. Why is this different?
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Actually, every new function that has been introduced caused worries about possible name collisions. The Perl 5 development crowd is very conscientious about backward compatibility. This concern and the strong culture of testing are probably the best things Perl has going for it (in addition to CPAN and its innate awesomeness, of course). Contrast this with Python (as an example), where the breaks they introduced going from 2.8 to 3.0 have caused large swaths of their community to have two different copies of the Python interpreter installed on their systems.
#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.
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In addition to what kennethk has said, there's another upside to these pragmas, especially explicit version requirements: they make your code fail gracefully on older versions.
Suppose that you do indeed use Perl 5.18 (or a later version), and you want to make use of all the features and goodies it has to offer, so you declare your intent with an explicit use 5.18.0;, and if someone then takes your script and attempts to run it on an older Perl version, they will get a message along the lines of "Perl v5.18.0 required--this is only v5.14.2".
Same for features. If you ask for a feature that doesn't (yet) exist, e.g. use feature qw/ponies/;, Perl will inform you that "Feature "ponies" is not supported by Perl 5.14.2". No code that fails with unrelated error messages, or (worse) silently, or that (even worse) silently does the wrong thing; no guessing, no headscratching, just a very clear declaration of the programmer's intent, the program's requirement, and the installed Perl's inability to meet the latter.
Even if it weren't for backwards compatibility, this is a feature. Forward compatibility (running newer scripts on older Perls) matters, too.
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