note Cristoforo Using this data (file 1 and file 2): <c>ID1 50 ID2 60 ID3 100 </c> <c>ID1 20 ID2 100 ID3 10 </c> I got the results: <c> C:\Old_Data\perlp>perl t9.pl o44.txt o55.txt ID1 50 20 ID2 60 100 ID3 100 10 C:\Old_Data\perlp></c> The code is: <c>#!/usr/bin/perl use strict; use warnings; my %data; while (<>) { # reads 2 files from @ARGV - filenames are on the command line my ($id, $val) = split; push @{ $data{$id} }, $val; } for my $id (sort keys %data) { print join("\t", $id, @{ $data{$id} }), "\n"; } </c> 1012948 1012948