use strict;
use warnings;
my $file_count=@ARGV; 
my %seen;
while(<>) {
	# File $ARGV saw line $_ at least once by file $ARGV
	$seen{$_}{$ARGV}++;  
}

while( my ($line, $by_whom) = each %seen) { 
	if ( $file_count == keys %$by_whom) {
		# Every file saw this line at least once
		print $line 
	}
}