it's much faster in many cases

And much slower in other cases. Empty prototyped subs which only return a constant and are know at compile time are constant folded at compile time. Unlike variables:

    $ perl -MO=Deparse -e 'sub CONST(){1;} my $CONST = 1; print CONST,
+ $CONST'
    sub CONST () { 1 }
    my $CONST = 1;
    print 1, $CONST;
    -e syntax OK
[download]

I can live with modules exporting constants as subs, and I can live with modules exporting constants as variables. But I would oppose changing core modules. Errno, Fcntl and Socket export constants as subs. No need for a change.