sub parse_bin {
   my $save = pos;
   my ($size_sz) = /\G \# (\d)        /xgc  or goto BACKTRACK;
   my ($bin_sz)  = /\G (\d{$size_sz}) /xgc  or goto BACKTRACK;
   my ($bin)     = /\G .{$bin_sz}     /xgcs or goto BACKTRACK;
   return $bin;

BACKTRACK:
   pos = $save;
   return ();
}

my $outputText = '';
for ($inputText) {
   pos = 0;

   for (;;) {
      if (my ($bin) = parse_bin()) {
         $outputText .= '<Binary block: '.length($bin).' bytes>';
         next;
      }

      if (/\G (.[^#]+ ) /xgcs) {
         $outputText .= $1;
         next;
      }

      last;
   }
}