for $i (1..$n)
     {
     for $j (1..$n)
          {

       ...stuff about $a[$i][$j]
          }
     }

##</code><code>##

foreach (keys %a)
     {
     ...stuff about $a{$_}
     }

##</code><code>##

for $i (1..$n)
     {
     }