#!perl

use strict;
use warnings;

# The final hash
my %all;

# process files one by one, gather lines from each
foreach (@ARGV) {
    my $hash = unique_in_file($_);
    $all{$_}++ for keys %$hash;
};

# print result
print "$_\n" for grep { 
    $all{$_} == @ARGV;  # don't hardcode the needed count, the number of files *will* change
} sort keys %all;

sub unique_in_file {
    my $fname = shift;

    # don't bother opening
    local @ARGV = ($fname);

    # do the same uniq(1) but for the current file
    my %uniq;
    while (<>) {
        chomp;
        # preprocess line here
        $uniq{$_}++;
    };

    # could've returned array as well
    return \%uniq;
};