for ( 0 .. 19 ) {
    print "$_\n";
}

##</code><code>##

for ( 0..7 )
{
  my $Col = $_;  #  Eek! Save $_ otherwise this value is lost
  for ( 0..7 )
  {
    # Do something in an 8x8 matrix.
    if ( $Matrix[ $Col ][ $_ ] == 4 ) ...
  }
}

##</code><code>##

for ( my $Col = 0; $Col < 8; $Col++ )
{
  for ( my $Row = 0; $Row < 8; $Row++ )
  {
    # Do something in an 8x8 matrix.
    if ( $Matrix[ $Col ][ $Row ] == 4 ) ...
  }
}