I don't think you get the intended result for 
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

[download]

It gives
2 -     1_a 31 1_b 20
where I'd expect
2 - 320 1_a 31 1_b 20

Also, there's usually a desire to only omit repeated header fields, yet your solution can omit any number of fields, even all of them.

Fix: 

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

[download]
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

[download]

In reply to Re^3: Loop, but print once. by ikegami
in thread Loop, but print once. by BioNrd

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