Perl Cd Bookshelf [Electronic resources] نسخه متنی

1.21.1. Problem

The use constant pragma is the easiest to use, but
has a few drawbacks. The biggest one is that it doesn't give you a
variable that you can expand in double-quoted strings. Another is
that it isn't scoped; it puts a subroutine of that name into the
package namespace.

The way the pragma really works is to create a subroutine of that
name that takes no arguments and always returns the same value (or
values if a list is provided). That means it goes into the current
package's namespace and isn't scoped. You could do the same thing
yourself this way:

sub AVOGADRO( ) { 6.02252e23 }

If you wanted it scoped to the current block, you could make a
temporary subroutine by assigning an anonymous subroutine to the
typeglob of that name:

use subs qw(AVOGADRO);
local *AVOGADRO = sub ( ) { 6.02252e23 };

But that's pretty magical, so you should comment the code if you
don't plan to use the pragma.

If instead of assigning to the typeglob a reference to a subroutine,
you assign to it a reference to a constant scalar, then you'll be
able to use the variable of that name. That's the second technique
given in the Solution. Its disadvantage is that typeglobs are
available only for package variables, not for lexicals created via
my. Under the recommended use
strict pragma, an undeclared package variable will
get you into trouble, too, but you can declare the variable using
our:

our $AVOGADRO;
local *AVOGADRO = \6.02252e23;

The third solution provided, that of creating your own little
tie class, might appear the most complicated, but
it provides the most flexibility. Plus you get to declare it as a
lexical if you want.

tie my $AVOGADRO, Tie::Constvar, 6.02252e23;

After which this is okay:

print "You need $AVOGADRO of those for guac\n";

But this will get you in trouble:

$AVOGADRO = 6.6256e-34; # sorry, Max