Anonymous Monk,
You likely want to be using a CSV parsing module like Text::CSV_XS or Text::x_SV, but for this example I will be using split. I have chosen to use split because I have made several assumptions about your problem.

Assumptions:

• Each record is contained on a single line
• Each record is pipe delimited and no field contains any imbedded delimiters
• Each record is comprised of 3 fields
• Preservation of record ordering is important
• 2 or more records with the first 2 fields in common are desired to be joined
• These records may be anywhere in the file and are not necessarily adjacent
• Joining records means concatenating the 3rd fields with commas in the order the records appeared in the file
• Concattenated records in the output will be identified by commas in the 3rd field. This assumes no commas appear in the 3rd field prior to merging.
• The joined record will appear at the first occurence in the output
• The machine running the program will have sufficient memory to hold required information in memory

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

my $input = $ARGV[0] || 'sample.txt';
open(my $fh, '<', $input) or die "Unable to open $input for reading: $
+!";
my %data;
while ( <$fh> ) {
    chomp;
    my @field = split /\|/, $_, 3;
    my $key = join '|', @field[0,1];
    $data{$key}{line} = $. if ! exists $data{$key};
    push @{ $data{$key}{records} }, $field[2];
}
for ( sort { $data{$a}{line} <=> $data{$b}{line} } keys %data ) {
    if ( @{ $data{$_}{records} } > 1 ) {
        my $field3 = join ',', @{ $data{$_}{records} };
        print join '|', $_, $field3;       
    }
    else {
        print join '|', $_, $data{$_}{records}[0];
    }
    print "\n";
}
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Please forgive me for the rather tedious solution. I wanted to point out the importance of clearly and concisely stating the problem and assumptions. Update: Simplified code and clarified assumptions