# delays determination of a scalar's value until needed

package Tie::Scalar::Once;

require Exporter;
@ISA = qw( Exporter );
@EXPORT = qw( delay );

use strict;

sub delay {
  tie $_[0], __PACKAGE__, \$_[0], $_[1];
}

sub TIESCALAR {
  my ($class, $var, $code) = @_;
  bless [ $var, $code ], $class;
}

sub FETCH {
  my ($self) = @_;
  my $val = $self->[1]->();
  untie ${ $self->[0] };
  ${ $self->[0] } = $val;
}

sub STORE {
  require Carp;
  Carp::croak("Can't store $_[1] to a constant");
}

1;
[download]

use Tie::Scalar::Once;

delay $x => sub { sqrt 2 };

# ...

if ($you_need_to) { print $x }
[download]

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;