If I want to use Perl 5.12 as if it were 5.10 then that is the exception to the rule

If that's true, you are an exception. Most people expect scripts written before Perl 5.12 was released to still run using perl 5.12 (for example).

I doubt it's true though. Contrary to what you say, I suspect you're not ok with new versions of Perl breaking your scripts.

if I have Perl 5.12 installed why should I have to specify: use 5.012;

You don't have to specify «use 5.012;». You just have to specify «use 5.012;» if you want backwards incompatible features of version 5.12. This enables the language to change without breaking backwards compatibility.

For example, «use feature qw( say );» (or «use 5.010;» or «use 5.012;») is required to use the say builtin since not requiring it would break the following program:

sub say { print "foo\n" }
say();
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