#!/usr/bin/perl

# http://perlmonks.org/?node_id=1142400

use strict;
use warnings;

# two strings

my $s1 = 'aaababbbababbbabababb';
my $s2 = 'aaababbbabaaababbabba';

($s1 ^ $s2) =~ /\0*/;
print $+[0], "\n";

# many strings (assumes no \n in strings)

my @many = qw(
  aaababbbababbbabababb
  aaababbbabaaababbabba
  aaababababaaababbabba
  aaababbbabaaaaabbabba
  );

join("\n", @many, '') =~ /^(.*).*\n(?:\1.*\n)*$/;
print length $1, "\n";

# also solves original problem :)

@many = qw(
  aaababbbababbbabababb
  aaababbbabaaababbabba
  );

join("\n", @many, '') =~ /^(.*).*\n(?:\1.*\n)*$/;
print length $1, "\n";
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As far as "fastest way", see Benchmark.pm :)