do you think writing products that don't use CPAN modules is bad coding practice ?

Depends. If there is a module that does exactly what is needed and the developer can live with the license then it should be used. Most modules on CPAN use the GPL or Artistic license so that shouldn't be a problem. Now if there is a module that doesn't quite fit the bill, but can be modified to do so, the situation changes. The reason and nature of the modification might be something the vendor considers a competitive advantage and not want to distribute. So the GPL and Artistic licenses could be stumbling blocks here. Rather than be in a grey area, the safe thing to do is custom development. This should be a last resort because it takes developer time that could be used for other features or fixes.

btw, the "runs out of the box..." excuse is just that. A lame excuse. The installation documentation should specify the pre-requisites. The installation routine should verify them. If pre-requisites are a concern, it's always nice to include them...

Have fun,
Carl Forde