my $SPACE  = q{ };
my $PRINTF = '%3s %3s %s %s %s %s';

# remember cells from previous row
my @prev_cells = ( $SPACE  ) x 6;

while (<DATA>) {
    # append a dash to the first column
    substr($_, 1, 0, ' -');

    my @cells = split /\t/, $_;
    
    # suppress display if same as above
    my @display_cells 
        = map {
            $_ eq shift @prev_cells ? $SPACE : $_
        } @cells;
    printf $PRINTF, @display_cells;
    @prev_cells = @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	312	1_a	30	1_b	20
2	312	2_a	22	2_b	12
2	310	1_a	31	1_b	20
2	310	2_a	21	2_b	12

##</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 - 312 1_a 30 1_b 20
        2_a 22 2_b 12
    310 1_a 31 1_b 20
        2_a 21 2_b 12