davido,
You are correct as the following modified code shows:

#!/usr/bin/perl
use strict;
use warnings;

use Benchmark qw(:all);

my $length = (int rand 100) + 100;
my @char = ( 'a' .. 'c' , '+' , 'd' .. 'f' );
my $string;

$string .= $char[ int rand 7 ] for  0 .. $length ;
my $count = -5;


my $results = timethese ( $count,
           {
               'transliterate' => sub { my $foo = $string; $foo =~ tr/
++/ /; },
               'substitution' => sub {  my $bar = $string; $bar =~s/\+
+/ /g; },
           },
           'none'
       );
cmpthese( $results ) ;
__END__
                  Rate  substitution transliterate
substitution  124624/s            --          -84%
transliterate 784349/s          529%            --
[download]

Cheers - L~R