http://www.perlmonks.org?node_id=338296
muba has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

I am writing a simple plugin system. I'm not sure whether I'm going to use it, but it's more like a way to explore Perl.

So, I have an array which contains packages names. Those packages should have a sub 'ReceiveMessage'.
A package called Communications contains a sub 'SendMessage'.
This SendMessage loops over the array to check which packages do have a ReceiveMessage and send a message to it. Currently, the array consists of one item: "Communications".

package Communications; sub SendMessage { my $msg = join("", @_); foreach my $cpack (@main::plugpack) { if (defined eval "${cpack}::ReceiveMessage") { eval "\&${cpack}::ReceiveMessage('$msg')"; } } } sub ReceiveMessage { print "Communications::ReceiveMessage('$_[0]')\n"; }
Of course I call the SendMessage sub once: &Communications::SendMessage("Hello, World!");
The strange thing is, I get output like I called SendMessage twice:
Use of uninitialized value in concatenation (.) or string at D:\device +\modem\internet\Xitami\cgi-bin\index.pl line 48. Communications::ReceiveMessage('') Communications::ReceiveMessage('Hello, World!')
It appears to me that ReceiveMessage is not only called when the message is sent, but also when SendMessage checks it's existance. How can I prevent this?

Update: just clarified the last paragraph a little
Update: removed line of code which was coded in a inconsistent style and only was there for debug purposes :)