$athlete[$i][$j++] = $bug;

##</code><code>##

$j = 0;
foreach my $bug (@row) {
  #here $j == 0; so the following line would be
  #$athlete[$i][0] = $bug;

  $athlete[$i][$j++] = $bug 

  #at this poing the $j == 1 because of the $j++
  #on the previous line.
  print "\$i\$j is assignt $i:$j\n";
  print "\@athlete[\$i,\$j] is assigned the value $athlete[$i][$j]\n";

  #on the previous line you $j == 1 so you are refering to
  #$athlete[$i][1] which is not the same as what you set
  #but rather the value you will set the next time through.
  . . .

##</code><code>##

print "\@athlete[\$i,\$j] is assigned the value $athlete[$i][$j-1]\n";