#!/usr/bin/perl

use strict;
use warnings;

exit unless @ARGV;

my %cache = do {local @ARGV = shift @ARGV; map {($_, 1)} <>};

while (@ARGV) {
    local @ARGV = shift @ARGV;
    %cache = map {$cache{$_} ? ($_, 1) : ()} <>;
}

print for keys %cache;

__END__
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Call it with the list of file names you want to find common lines in.