why do i have to specify "use feature :5.1x" even when my installed perl is 5.14?

Question

I have 5.14.2 installed in my machine but when i try to execute a print statement with say keyword it gives me error.

I have go with the use keyword to make the program run without error.

#!/usr/bin/perl
use feature ':5.10';
say "hello";

5.14.2 is latest when compared to 5.010 so it should have all those features enabled by default right ? then what is the point in specifying the version using use keyword ?

Answer 1

Perl attempts to maintain backward compatibility. It's quite possible that existing scripts might have subroutines named say. There is considerable on-going discussion of whether or not some future version of Perl should stop these efforts and streamline its internals. See, for instance, naming and numbering perl .

Answer 2

It prevents conflicts with existing programs written in Perl.

For example, say I wrote a program for Perl 5.6 which defined a subroutine called say.

use strict;
use warnings;
sub say { print 1; }
say();

That works fine (outputting 1), and it still works in perls that include the say feature.

Now let's enable the native say and see what happens:

use v5.14;
use strict;
use warnings;
sub say { print 1; }
say();

Now it falls over with *Use of uninitialized value $_ in say at - line 5.*

You need to use feature or use v5.xx so that the new features can be loaded safely, i.e. when the author knows he wants to use them.