#!/usr/bin/env perl
use warnings;
use strict;

my $prev_first  = 0;
my $prev_second = 0;    # you need to make use of this too
while (<DATA>) {
    chomp;
    my ($first, $second, @data) = split /\t/;
    if ($first == $prev_first) {
        print "    $second @data\n";
    }
    else {
        print "$first - $second @data\n";
        $prev_first = $first;
    }
}

__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
    300 2_a 22 2_b 12
    320 1_a 10 1_b 30
    320 2_a 32 2_b 22
2 - 312 1_a 30 1_b 20
    312 2_a 22 2_b 12
    310 1_a 31 1_b 20
    310 2_a 21 2_b 12