I tried to gather some of the ideas above and merge them into one benchmarked thing. Also, I benchmarked replacing the split // and chop $var while $var with unpack. Anyway, hope you enjoy.

#!/usr/bin/perl

use Benchmark qw(timethese cmpthese);

my $string1   = 'This is a test of the emergency system!';
my $string2   = 'This is a test of the emergency broadcast system!';
my $template1 = "a1" x (length $string1);
my $template2 = "a1" x (length $string2);

print &xor_o,    "\n",
      &split_o,  "\n",
      &unpack_o, "\n",
      &chop_o,   "\n",
      &chop_o2,  "\n";

my $results = timethese( 100000, {
                                   XOR    => \&xor_o,
                                   SPLIT  => \&split_o,
                                   UNPACK => \&unpack_o,
                                   CHOP   => \&chop_o,
                                   CHOP2  => \&chop_o2
                                 }
                       );
cmpthese($results);

sub xor_o {
    my $xor   = $string1^$string2;
    return ($xor =~ tr/\0//)/length($string1);
}


sub split_o {
    my @string1 = split //, $string1;
    my @string2 = split //, $string2;
    my $score   = 0;
    my $length  = scalar @string1;

    for (my $i = 0; $i < $length; $i++)
    {
        $score += ($string1[$i] eq $string2[$i]);
    }

    return $score/$length;
}

sub unpack_o {
    my @string1 = (unpack $template1, $string1);
    my @string2 = (unpack $template2, $string2);
    my $score   = 0;
    my $length  = scalar @string1;

    for (my $i = 0; $i < $length; $i++)
    {
        $score += ($string1[$i] eq $string2[$i]);
    }

    return $score/$length;
}

sub chop_o {
    my @string1 = ();
    my @string2 = ();

    my $score   = 0;
    my $rstring1 = scalar reverse $string1;
    my $rstring2 = scalar reverse $string2;
    push @string1, (chop $rstring1) while $rstring1;
    push @string2, (chop $rstring2) while $rstring2;
    my $length  = scalar @string1;

    for (my $i = 0; $i < $length; $i++)
    {
        $score += ($string1[$i] eq $string2[$i]);
    }

    return $score/$length;
}

sub chop_o2 {
    my $str1    = $string1;
    my $str2    = $string2;
    my $length  = length $string1;
    my $score;

    $score += (chop $str1 eq chop $str2) while $str1;
    return $score/$length;
}


Benchmark: timing 100000 iterations of CHOP, CHOP2, SPLIT, UNPACK, XOR
+...
      CHOP: 43 wallclock secs (37.29 usr +  0.05 sys = 37.34 CPU) @ 26
+78.09/s (n=100000)
     CHOP2:  9 wallclock secs ( 7.34 usr +  0.03 sys =  7.37 CPU) @ 13
+568.52/s (n=100000)
     SPLIT: 47 wallclock secs (38.76 usr +  0.07 sys = 38.83 CPU) @ 25
+75.33/s (n=100000)
    UNPACK: 34 wallclock secs (29.51 usr +  0.03 sys = 29.54 CPU) @ 33
+85.24/s (n=100000)
       XOR:  2 wallclock secs ( 0.92 usr +  0.00 sys =  0.92 CPU) @ 10
+8695.65/s (n=100000)
           Rate  SPLIT   CHOP UNPACK  CHOP2    XOR
SPLIT    2575/s     --    -4%   -24%   -81%   -98%
CHOP     2678/s     4%     --   -21%   -80%   -98%
UNPACK   3385/s    31%    26%     --   -75%   -97%
CHOP2   13569/s   427%   407%   301%     --   -88%
XOR    108696/s  4121%  3959%  3111%   701%     --
[download]

Update: My apologies for the length @array lines above. That's what I get for yanking and putting. :) Thanks to those who caught it. New benchmarks are now shown.

Update2: I've also added the chop2 implemented in the comment below.

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