Your code match the first occurrence of [a-z] in every element in @ARGV, and returns occurrences that occur at least as many times as elements in @ARGV. Sorry -- this is nothing like the original requirements.

P.S. Helpful hint: Even if you are only determining whether the concept you are trying for is correct, any posted code that is not in the form of pseudo-code should be tested. You would have found that your code did not meet the requirements without having had to ask. Cheers.

UPDATE: As per the followup by Anonymous Monk, I did miss the inner while(){}. The only line that appears to still be wrong is the line that reads "$found{$1} = $i + 1;". The effect appears to be that only characters that show up in the last element of @ARGV will be returned. A small adjustment that would perhaps allow this code to work would be to replace the faulty line with "$found{$1}++;". One good addition that this code suggests is an in-case-sensitive match. In order for this to work however, the faulty line will need to read "$found{lc $1}++;" to ensure that the correct hash element is incremented.

As far as I understand it, for a character to be common to all words being scanned, it has to occur at least once in each word. If @ARGV contains 3 words to be scanned, for example, then we are only interested in characters than occur a minimum of once per word for all three words, or three times. That's why when a character is found in a given word for the first time, it's "score" is incremented by 1 to bring the cumulative score to the word number +1 (since arrays start at 0). Other occurences of the same character in the same word are effectively ignored since we don't care. Only characters that score 3 are returned. I think this theory does satisfy the original requirements actually, even if the code itself is buggy...

#!/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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