Am I to assume that elements 0-69 and 71-120 are undefined?

Yes. If they were undefined, they will remain undefined. 

$ perl -le'
    my @a;
    print 0+@a;
    print defined($a[0]) || 0;
    $a[70] = "a";
    print 0+@a;
    print defined($a[0]) || 0;
'
0    0 elements in the array
0        and the first one isn't defined.
71   71 elements in the array
0        and the first one still isn't defined.

[download]

My problem is that not defined ($array[$number] returns false

Then $array[$number] is defined. It was given a value somewhere else, possibly through autovivification. 

$ perl -le'
    my @a;
    print defined($a[0]) || 0;
    $a[0][0];
    print defined($a[0]) || 0;
'
0
1

[download]

[@{$x->[3]}>0 gives] Can't use an undefined value as an ARRAY reference

Then $array[$number][3] is not defined.

Update: Added code blocks.

In reply to Re: Erroneous defined detection of array elements by ikegami
in thread Erroneous defined detection of array elements by Anonymous Monk

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