One way to do it is to create state variables to remember previous lines. This gets you half the way there ... but, you get the idea. 
#!/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

[download]

This prints: 

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

[download]

I used actual tab characters in the __DATA__, not "\" followed by "t".

In reply to Re: Loop, but print once. by toolic
in thread Loop, but print once. by BioNrd

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