Other monks have already explained what's going on. Let me point to efficiency of the solutions:

Note that the transliteration is much faster than the other option. Even when the character is variable and we have to use string eval (whip! whip!), it's much faster.

Instead of using substitution with length, you can use global substitution only, as it returns the number of replacements in scalar context. But it's still slower than transliteration: 

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

use Benchmark qw{ cmpthese };

my $orig = 'Just another Perl hacker,' x 100;
my $str  = $orig;
my $char = 'r';
my $q    = quotemeta $char;

sub transliteration {
    my $count = eval "\$str =~ tr/$q//"
}

sub length_subst {
    my $count = length( $str =~ s/[^$q]//rg )
}

sub subst {
    my $count = $str =~ s/$q/$char/g
}

transliteration() eq length_subst() or die 'Different t-ls';
transliteration() eq subst()        or die 'Different t-s';
$orig eq $str or die 'Changed';

cmpthese(-3, {
   transliteration => \&transliteration,
   length_subst    => \&length_subst,
   subst           => \&subst,
});

__END__
                    Rate    length_subst           subst transliterati
+on
length_subst      2833/s              --            -91%            -9
+7%
subst            30244/s            968%              --            -7
+0%
transliteration 102423/s           3515%            239%              
+--

[download]

Update: Introduced quotemeta to transliteration, too. It didn't change the results significantly.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Re: Confused by RegEx count by choroba
in thread Confused by RegEx count by Melly

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.