Here is something along the lines of what monkfan had probably written

#!/usr/bin/perl --
use strict;
use warnings;
use Benchmark qw(cmpthese);
our $data;
print "\n$]\n";
{
    my $s1 = 'AAAAA';
    my $s2 = 'ATCAA';
    print join ' ', RoyJohnson500332($s1,$s2)," ";
    print join ' ', monkfan500235($s1,$s2)," ";
    print join ' ', BrowserUk500244($s1,$s2)," ";
    print join ' ', inman500994($s1,$s2)," ";
    print "\n";
}


for my $range ( 5, 1_000 , 2_000 , 10_000 , 100_000 ){
    $data = join '',map { ( qw' T A C G ' )[ $_ % 4 ] } 0 .. $range;
    my $s1 = $data.'AAAAA';
    my $s2 = $data.'ATCAA';
    
    print "## Length $range   ", "##" x 11, "\n";
    cmpthese (-3, {
        Mine => sub { monkfan500235($s1,$s2); return },
        BUk => sub { BrowserUk500244($s1,$s2); return },
        inman => sub { inman500994($s1,$s2); return },
        RJ => sub { RoyJohnson500332($s1,$s2); return },
    });
    print "\n";
}

sub monkfan500235 {
     my ($k,$l) = @_;
     my $len = length ($k);
     my $num_mismatch = 0;

     for (my $i=0; $i<$len; $i++)
     {
      ++$num_mismatch if substr($k, $i, 1) ne substr($l, $i, 1);
     }
     return $num_mismatch;
}

sub BrowserUk500244 { length( $_[ 0 ] ) - ( ( $_[ 0 ] ^ $_[ 1 ] ) =~ t
+r[\0][\0] )  }
sub RoyJohnson500332 {
  my ($k, $l) = @_;
  my $diff = $k ^ $l;
  my $num_mismatch = $diff =~ tr/\0//c;
}

sub inman500994 {
    return ($_[0] ^ $_[1]) =~ tr/\001-\255//;
}

__END__

5.012002
2  2  2  2
## Length 5   ######################
           Rate  Mine    RJ   BUk inman
Mine   127809/s    --  -78%  -85%  -87%
RJ     580454/s  354%    --  -33%  -43%
BUk    872298/s  582%   50%    --  -14%
inman 1012310/s  692%   74%   16%    --

## Length 1000   ######################
          Rate   Mine     RJ    BUk  inman
Mine    1862/s     --   -99%   -99%   -99%
RJ    176683/s  9389%     --   -18%   -21%
BUk   216727/s 11540%    23%     --    -4%
inman 224899/s 11979%    27%     4%     --

## Length 2000   ######################
          Rate   Mine     RJ    BUk  inman
Mine     934/s     --   -99%   -99%   -99%
RJ    100661/s 10677%     --   -17%   -19%
BUk   120656/s 12818%    20%     --    -2%
inman 123544/s 13127%    23%     2%     --

## Length 10000   ######################
         Rate   Mine     RJ  inman    BUk
Mine    187/s     --   -99%   -99%   -99%
RJ    22959/s 12181%     --   -16%   -18%
inman 27435/s 14575%    19%     --    -2%
BUk   27976/s 14865%    22%     2%     --

## Length 100000   ######################
        Rate   Mine     RJ  inman    BUk
Mine  18.7/s     --   -99%   -99%   -99%
RJ    2085/s 11031%     --   -21%   -22%
inman 2651/s 14054%    27%     --    -1%
BUk   2667/s 14136%    28%     1%     --
[download]

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