This is how I'd split the arguments up. Comments in the code should provide enough explanation.

my $ArgV = "a1:b1:c1:d1;:a2:b2:c2;:a3;:a4;:a5:b5;:a6:b6:c6;:a7:b7";
my @Array;
my ( $ThisRow, $ThisCol );

#  Split data into rows on the ';' character.

$ThisRow = 0;
foreach my $Row ( split ( ';', $ArgV ) )
{
  # Split the columns on the ':' character.

  $ThisCol = 0;
  foreach my $Element ( split ( ':', $Row ) )
  {
    #  Skip blank elements; then insert data into the appropriate
    #  Row, Column element then move on to the next column.

    next if ( $Element eq "" );
    $Array[ $ThisRow ][ $ThisCol++ ] = $Element;
  }

  #  Move on to the next row.

  $ThisRow++;
}

#  Now print out the array to show that we've got 
#  the right stuff in the right spot.

for ( my $RowNow = 0; defined ( $Array[ $RowNow ] ); $RowNow++ )
{
  for ( my $ColNow = 0; defined ( $Array[ $RowNow ][ $ColNow ] ); $Col
+Now++ )
  {
    print "[ $RowNow ][ $ColNow ]: $Array[ $RowNow ][ $ColNow ]. ";
  }
  print "\n";
}
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