This should get you started:

$ perl -e'
$x = <<STUFF;
1\t300\t1_a\t30\t1_b\t20
1\t300\t2_a\t22\t2_b\t12
1\t320\t1_a\t10\t1_b\t30
1\t320\t2_a\t32\t2_b\t22
2\t312\t1_a\t30\t1_b\t20
2\t312\t2_a\t22\t2_b\t12
2\t310\t1_a\t31\t1_b\t20
2\t310\t2_a\t21\t2_b\t12
STUFF
open my $fh, "<", \$x or die $!;
my @last;        
while ( <$fh> ) {
    my @fields = split /\t/;
    for my $i ( 0 .. $#fields ) {
        if ( $fields[ $i ] eq $last[ $i ] ) {
            $fields[ $i ] = "";
            }
        else {
            last;
            }
        }               
    print join "\t", @fields;
    @last = split /\t/;
    }
'
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
[download]