Here's a version using List::Util. Pretty straight-forward.

#! /usr/bin/perl

use strict;
use warnings;

use List::Util q(first);

sub csp {
  my ($n, $s) = (-1, shift // return);

  while (++$n < length($s)) {
    my $c = substr($s,$n,1);
    last if first { substr($_,$n,1) ne $c } @_;
  }

  substr($s,0,$n);
}

print csp(qw(ahhaaaaa aha ahem));
[download]

But sorting the strings by length could still prove beneficial, esp. if the data is already thus structured.