Dear Wise Monks,

I recently ran into something strange with the Tk package and I wondered if anyone has an explanation. Somehow, Tk helps another package that is not an Exporter to export its constants.

Here is the example:

This package is trying to export some constants, but it fails to inherit from Exporter, so you would think it could never work. 

# Constants.pm
package Constants;

use constant XX       => 0;
use constant YY       => 1;

our @EXPORT = qw(XX YY);
1;

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This program will try to use the constants: 
# caller.pl
use strict;
use warnings;
use Tk;
use Constants;

print "sum =",XX + YY,"\n";

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Here is the strange thing. With the "use Tk" in place, the program works and prints "sum =3". Without it, the program fails with the expected 'Bareword "XX" not allowed' messages. This is perl version 5.6.1.

Why does Tk have this accidental side-effect? I think it may have something to do with the following line in the Tk.pm header: 

use base qw(Exporter DynaLoader);

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Does "use base" have side-effects on other packages, causing them to become subclasses of the base list too? This seems quite strange and a potential source of errors. Any ideas?

In reply to Tk helps another package to export? by tall_man

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