Does a bit less work:

sub popfileb {
    #set first row;
    substr $aox[0], 0, 11, 'n' x 11;

    foreach my $yb ( 1 .. 10 ) {
        foreach my $xb ( 0 .. 10 ) {
            #set increasing rows;
            substr $aox[$yb], $xb, 1, 'a';
        }

        $aod[$yb] = $aox[$yb] . "\n";
    }
}
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I disapprove of doing IO in a function that has nothing to do with IO, though. «$aod[$yb] = $aox[$yb] . "\n";» shouldn't be in that function.