The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


Tie::Scalar::Decay - Scalar variables that decay


  use Tie::Scalar::Decay;

  tie my $scalar, 'Tie::Scalar::Decay', (
    VALUE => 32,
    FUNCTION => '$value-=1',
    PERIOD => 1

  while($scalar>0) {
    print "$scalar\n";


This module allows you to tie a scalar variable whose value will change regularly with time. The default behaviour is for numeric values to halve every time period (a la radioactive decay) and for non-numeric values to be unchanged.

You can specify a custom decay function if you wish.

The following named parameters are supported:


Use PERIOD to specify the time interval for changes. This can be either a numeric value, in which case it is taken to be a number of seconds, or the following forms are also accepted:

    30s                    every thirty seconds
    10m                    every ten minutes
    1h                     every hour
    1d                     every day

Assigning a value to the variable causes the timer to be reset to zero, so if at t=0 you set a value of 5 with a timeout of thirty seconds, then wait twenty seconds and set the value again, then it will not decay until t=50. The default is a somewhat arbitrary 5 seconds.


Using the VALUE hash key, you can specify an initial value for the variable. Defaults to undef.


This is how you can define your own custom decay functions. This can either be a string (in which case it is evalled as necessary, and should alter the variable $value as it sees fit) or it can be a coderef, in which case the subroutine is called as necessary, with the current value as a parameter. The sub is expected to return the new value. If you don't specify a function, it defaults to one which halves the value if it is numeric, or leaves it as it is otherwise.

Note that whilst it may appear that your FUNCTION gets called every PERIOD, it isn't really. In reality, the value stored in the tied scalar remains constant, and every time you try to read its value, a temporary variable is created and FUNCTION gets called the appropriate number of times before that is returned, thus generating the right illusion. Therefore, your FUNCTION should not depend on such things as the absolute time.


Plenty, no doubt. Please tell me if you find any.

One caveat is that it relies on Time::HiRes. If your system doesn't support this, then you can comment it out of the module and it'll still mostly work. You may get hurt by sub-second periods (they still work but the granularity of the timer is only a second) and very occasionally by boundary conditions if the load on your machine is high. Timing things with 1-second accuracy blows goats.


David Cantrell <>

This module was inspired by Marcel Grunauer's Tie::Scalar::Timeout.


Copyright 2001 David Cantrell.

This module is licensed under the same terms as perl itself.


Tie::Scalar(3pm), Tie::Scalar::Timeout(3)