The most likely situation is that you are trying to dereference a row before it is initialized: demonstrated by the code: 

#!/usr/bin/perl
use strict;
use warnings;

my @row = ( [1] ) x 3;

print @{$row[2]}, "\n";
print @{$row[3]}, "\n";

[download]
Are you sure @aob contains what you think it does? See How can I visualize my complex data structure?. You can also wrap your dereference in an eval to catch the case where it fails, like: 

sub popnum3 {
    use Data::Dumper;
    my ( $x, $y, $z ) = @_;
    my @row  = eval{@{$aob[ $y - 1 ]}} or die "$x\n$y\n$z\n", Dumper $
+aob[ $y - 1 ];
    my $mean = mean(@row);
    $aob[$x][$y] = $mean * ( 1 + $z );
    return $aob[$x][$y];
}

[download]

In reply to Re: undefined value in array reference by kennethk
in thread undefined value in array reference by Dandello

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