Tracing 

• Turning on call tracing
• In the debugger you can turn it on with the command t
• Or you can turn it on and off in your code

$DB::single = 1 if $DEBUG;
my $equatorial_radius = 6378.135; # equatorial radius, kilometres
my $polar_radius      = 6356.750; # polar radius, kilometres

# quadratic mean radius
my $qr = sqrt(((3 * ($equatorial_radius**2)) + ($polar_radius**2))/4);

$DB::trace = 0;