You can store the data as a HoA (Hash of Array). The keys are "type pos" and the value is an array of ids. The number of ids in the array gives you the count. The ids themselves allow the original data line to be reconstructed.

The OP said, "Normally I would simply use an incrementing hash to detect duplicate entries, looking for any values >1". The code below is essentially that idea, except rather than incrementing a simple scalar, a new element is pushed onto an array. 

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

<DATA>; #throw way first line

my %ids;  #Hash of Array  "$type $pos" => @ids

while (<DATA>)
{
  my ($id,$type,$pos) = split;
  push @{$ids{"$type $pos"}}, $id; 
}

foreach my $key (sort keys %ids)
{
    next if @{$ids{$key}} == 1;
    
    foreach my $id (@{$ids{$key}})
    {
       print "$id $key\n";
    }
}

=prints:
2 1 11
3 1 11
5 2 5
6 2 5
=cut


__DATA__
ID   Type      Pos
1    1    10
2    1    11
3    1    11
4    1    15
5    2    5
6    2    5
7    2    7

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In reply to Re: Identifying duplicates in array or hash based on a subset of data by Marshall
in thread Identifying duplicates in array or hash based on a subset of data by Anonymous Monk

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