Use the Exporter module. Here's a sample:
package Constants;
use Exporter;
$Pi = 3.14;
$Avagadros = 6.02e23;
push @EXPORT, $Pi, $Avagadros; # These variables I want to export automatically
$R = 8.31;
$STP = 22.4;
push @EXPORT_OK, $R, $STP; # This variable is exported only on request, when you...