To end with that, the problem is effectively a lib one. PHP module has is own libgd version which is used when configure is invoked with '--with-gd'. So, when a CGI Perl invokes GD, some strange things occures.

To avoid that without suppress PHP or PHP gd support, it's necessary to pass to configure '--with-gd=<path>' where <path> points to system directory where is the gd include directory and the libgd file. For me, I've used '--with-gd=/usr'.

And no more strange things !

Thanks again

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poum