#! /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";