I have no idea why it generates that weird data structure!. It is not as though it is even consistent as the first value is undef (as you might expect) but then the other ones......are references to this undef value. Looks a bit like a {cough} feature to me. This may be a feature either of perl or Data::Dumper. As to why the grep then modifies this matrix just by looking at it -> GOK. Anyway I think this is probably the matrix you wanted in @taulell.

sub inicialitzacions
{
    $torn='B';
    @taulell = ([],[],[],[('','','','B','N')],[('','','','N','B')]);
    @pos{'x','y'}=(2,2);
}
[download]

cheers

tachyon

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