Just off the top:

foreach (@stuff)
{
     my ($a,$b,@stuff) = split (/:/);

     push (@{$data{$a}{$b}}, @stuff);
}
[download]

Or, if you are working with unique A:B pairs, you could do something like this:

foreach (@stuff)
{
     my ($a,$b,@stuff) = split (/:/);

     $data{$a}{$b} = [ @stuff ];
}
[download]

Whatever method you choose to put the stuff in there, you are going to have to get it out. The tricky part is figuring out how big your grid is, no? If each row can have holes, you are going to have to find all possible columns up front, like so:

my %column_width;

foreach my $row (keys %data)
{
     foreach my $column (keys %{$data{$row}})
     {
          my $data_length = length($data{$row}{$column});

          if (!exists($column_width{$column})
          {
               # Make sure the label fits
               $column_width{$column} = length($column);
          }

          if ($column_width{$column} < $data_length)
          {
               $column_width{$column} = $data_length;
          }
     }
}

# Order the columns accordingly. You may need a better sort.
my @columns = sort keys %column_width;

print join (" | ",
           sprintf ("%15s", ""),
           map {
                sprintf ("%*s", $column_width{$_}, $_)
           } @columns;
),"\n";

foreach my $row (sort keys %data)
{
     my $continued;
     my $line = 0;

     do {
          $continued = 0;
          print join (" | ", 
                # Only title the first line (0th)
                sprintf ("%15s", $line? '' : $row),

                map {
                     $continued++ if (defined($data{$row}{$_}[$line+1]
+));

                     sprintf ("%*s", $column_width{$_},
                           defined($data{$row}{$_}[$line])?
                             $data{$row}{$_}[$line] : '');
               } @columns
          ), "\n";
          $line++;
     } while ($continued);
}
[download]

For simplicity, I have chosen 15 as an arbitrary width. It is trivial to calculate this, as has been done for each column.

Testing and debugging, as they say, is left as an exercise for the reader.