#!/usr/bin/perl

use strict;
use warnings;

my $charClass = '[a-z]'; #change as desired
my @commonChars = compareChars();
print "Num common chars: ",scalar @commonChars,"\n";

### sub assumes case insensitivity is appropriate ###
sub compareChars {
	my $requiredCount = @ARGV;
	my %found;
	my @answer;
	for my $word ( @ARGV ) {
		my %nonredundantChars = map { $_ => 1 } split //, $word;
		$word = join '', keys %nonredundantChars;
		$found{lc $1}++ while ( $word =~ m/($charClass)/gi )
	} 
	while ( ( my $char, my $count ) = each %found ) {
		push @answer, $char if $count == $requiredCount;
	}
	return @answer;
}