#!/usr/local/bin/perl -w

use strict;

my $desc = "I %2Blike %3A cheese";
my ($str, @ob);

print "TEST: ",test(),"\n";<br>

sub test {
    $_ = $desc;
    while (/%[0-9A-Za-z]{2}/) {
        $_ = $&;
        /[0-9A-Za-z]{2}/;
        $ob[0] = hex($&);
        $str = pack("C*", @ob);
        $desc =~ s/%[0-9A-Za-z]{2}/$str/;
        $_ = $desc;
        }
    return $desc;
}

returns:

TEST: I +like : cheese


##</code><code>##

...
'myway' => sub {
    $_ = $str;
    $name = $str;
    while (/%[0-9A-Fa-f]{2}/) {
       $_ = $&;
...

##</code><code>##

Benchmark: timing 500000 iterations of myway, regexpway...
     myway: 162 wallclock secs (162.07 usr +  0.00 sys = 162.07 CPU)
 regexpway: 58 wallclock secs (58.52 usr +  0.01 sys = 58.53 CPU)