Also, a slightly more general example solution taking n>=2 files on the cmd line and listing for each those entries which are in the other ones too, and the actual files these entries are in. Mostly self explanatory, I suppose:

#!/usr/bin/perl

use strict;
use warnings;

die "Usage: $0 <file1> <file2> [<files>]\n"
  unless +(my @files=@ARGV) >= 2;

my %have;
chomp, $have{$ARGV}{ (split /,/)[0] }=1 while <>;

$\="\n";
for (@files) {
    {
        my $sep=':' x length;
        print "$sep\n$_\n$sep\n";
    }
    my @rest=do {
        my $ex=$_;
        grep $_ ne $ex, @files;
    };
    for my $k (sort keys %{ $have{$_} }) {
        my @dups=grep $have{$_}{$k}, @rest;
        print "$k => @dups" if @dups;
    }
    print '';
}

__END__
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(this assumes all of the files it is given are in the same format as your "file1". Modify at will!)