rand(0 .. $total)
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should be

rand($total+1)
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Actually, it probably be should be

rand(0+@aob)
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If any element of @aob can be picked, why rely on some external value.

(Technically, the "0+" is unnecessary because of rand's prototype, but that's fairly obscure.)

By the way, $cnr is unneeded. I also got rid of q{} because it just doesn't stand out as much as the default quotes.

while (1) {
    my $xda = int rand( 0+@aob );
    if ( $aob[$xda][ $y - 1 ] ne "") {
        $aob[$x][$y] = $z + $aob[$xda][ $y - 1 ];
        last;
    }
}
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