#!/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;
}
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