tux242,
As I explained in the CB, I understand how frustrating it can be just getting started. If you can put in clear and concise terms what you are trying to accomplish, we will be much more able to help you. What I discerned from the CB is this:

"You are trying to get a list of items that appears in both file1 and file2, concatenate them together, and disregard the rest".

#!/usr/bin/perl -w
use strict;

my %data;
my @files = qw(file1.dat file2.dat);

for my $file ( @files ) {
    parse_file( $file, \%data );
}

for my $key ( grep  $data{$_}->[1] > 1 , keys %data ) {
    print join ':' , $key ,  $data{$key}->[0];
}

sub parse_file {
    my ($file, $data) = @_;
    open (INPUT, $file) or die "Unable to open $file : $!";
    while ( <INPUT> ) {
        my ($key, $value) = split /:/ , $_ , 2;
        $data->{$key}[0] = $value;
        $data->{$key}[1]++;
    }
}
[download]

Tailor as needed.
Explanation:

Cheers - L~R