The answers are:
1) Perl has a special value for undef, called SV_UNDEF. Rather than create a separate 'undef' for each uninitialized value, Perl stores a pointer to this special undef value. Thus, the output from Data::Dumper is, in a way, correct; all of the uninitialized array elements are pointing to the same value, SV_UNDEF.
2) Auto-vivification. grep { 1 } @$_ for @matrix; treats each element of @matrix as an array reference, so the uninitialized elements are autovivified.
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