Always more than one way to skin a cat.
Here's another simple way.

#!/usr/bin/perl -w

use strict;
my %gbhash;

open(DATA, '/usr/local/fun/data.txt') or die $!;
while (<DATA>) {
  chomp;
  my ($key,$val) = split;
  if (exists $gbhash{$key}) {
    $gbhash{$key} = $gbhash{$key} . '-' . $val;
  } else { 
    $gbhash{$key} = $val;
  }
}
close DATA;

#Now print em
foreach my $skey (sort {$a<=>$b} keys %gbhash) {
  print "$skey $gbhash{$skey}\n";
}

------
DATA (data.txt):
1 0
1 1
2 0
3 1 
4 3
4 6
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nandeya