The following isn't efficient because it keeps re-matching the entire string, but this won't matter unless your strings are very long. It is simple.

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

my $str = "word1, word2, #18abcdefgh ,word4, #24qwer, word5";

print "Before: $str\n";

while($str =~ m/(.*?)#(\d)(\d)(.*)/) {
    $str = $1 . "<Binary block($2): $3 bytes>" . substr($4, $3);
}

print "After:  $str\n";
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Which produces:

Before: word1, word2, #18abcdefgh ,word4, #24qwer, word5
After:  word1, word2, <Binary block(1): 8 bytes> ,word4, <Binary block
+(2): 4 bytes>, word5
[download]

I wondered about the first digit after the '#' so I captured that and put it into the substitution also.

update: removed unnecessary capture from the RE.

update2: A better alternative and comparison:

#! /usr/local/bin/perl
use strict;
use warnings;
use Benchmark qw(cmpthese);

my $str = "word1, word2, #18abcdefgh ,word4, #24qwer, word5";

print "Before: $str\n";

while($str =~ m/#(\d)(\d)/g) {
    substr($str, pos($str) - 3, $2 + 3, "<Binary block($1): $2 bytes>"
+);
}

print "After:  $str\n";

my $start = "word1, word2, #18abcdefgh ,word4, #24qwer, word5";


print "\n\n";
cmpthese(
    -10, {
        're-match whole string' => sub {
            my $str = $start;
            while($str =~ m/(.*?)#(\d)(\d)(.*)/) {
                $str = $1 . "<Binary block($2): $3 bytes>" . substr($4
+, $3);
            }
        },
        'match #\d\d' => sub {
            my $str = $start;
            while($str =~ m/#(\d)(\d)/g) {
                substr($str, pos($str) - 3, $2 + 3, "<Binary block($1)
+: $2 bytes>");
            }
        },
    },
);
[download]

Before: word1, word2, #18abcdefgh ,word4, #24qwer, word5
After:  word1, word2, <Binary block(1): 8 bytes> ,word4, <Binary block
+(2): 4 bytes>, word5


                          Rate re-match whole string           match #
+\d\d
re-match whole string  75230/s                    --                  
+-28%
match #\d\d           104315/s                   39%                  
+  --
[download]