Using this data (file 1 and file 2):

ID1     50
ID2     60
ID3     100
[download]

ID1     20
ID2     100
ID3      10
[download]

I got the results:

C:\Old_Data\perlp>perl t9.pl o44.txt o55.txt
ID1     50      20
ID2     60      100
ID3     100     10

C:\Old_Data\perlp>
[download]

The code is:

#!/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";    
}
[download]