1	300	1_a	30	1_b	20
1	300	2_a	22	2_b	12
1	320	1_a	10	1_b	30
1	320	2_a	32	2_b	22
2	320	1_a	31	1_b	20
2	320	2_a	21	2_b	12

##</code><code>##

use strict;
use warnings;

my $PRINTF  = '%3s %3s %s %s %s %s';
my $HEADERS = 2;

my @prev_cells;

while (<DATA>) {
    # Input data.
    my @cells = split /\t/, $_;

    # Format the data for printing.
    $cells[0] .= ' -';

    # Remove redundant headers.
    my @display_cells = @cells;
    if (@prev_cells) {
       for (@display_cells[ 0 .. $HEADERS-1 ]) {
          last if $_ ne shift @prev_cells;
          $_ = '';
       }
    }
    @prev_cells = @cells;

    # Output data.
    printf $PRINTF, @display_cells;
}

__DATA__
1	300	1_a	30	1_b	20
1	300	2_a	22	2_b	12
1	320	1_a	10	1_b	30
1	320	2_a	32	2_b	22
2	320	1_a	31	1_b	20
2	320	2_a	21	2_b	12
2	320	2_a	33	2_b	44

##</code><code>##

1 - 300 1_a 30 1_b 20
        2_a 22 2_b 12
    320 1_a 10 1_b 30
        2_a 32 2_b 22
2 - 320 1_a 31 1_b 20
        2_a 21 2_b 12
        2_a 33 2_b 44