for (my $pos = length($text); $pos ; --$pos)
{
   my $char = substr($text, $pos-1, 1);
   if ($char =~ /\S/ and (int rand 2)) # 0 or 1
   {
       substr($text, $pos-1, 1) = "$opening_tag$char$closing_tag";
   }
}

##</code><code>##

$text =~ s/(\S)/(int rand 2) ? "$opening_tag$1$closing_tag" : $1/ge;

##</code><code>##

my $tag_is_open = 0;
for (my $pos = length($text); $pos > 0; )
{
   my $char = substr($text, -$pos, 1);
   if ($char =~ /\S/ and (int rand 2)) # 0 or 1
   {
       if ($tag_is_open) {
           substr($text, -$pos, 1) .= $closing_tag;
           $pos-=2; # After closing a tag, skip at least one char
       }
       else {
           substr($text, -$pos, 0) = $opening_tag;
       }
       $tag_is_open ^= 1; # Toggle                            
   }
   else { --$pos }
}
$text .= $closing_tag if $tag_is_open;