$string = "
hello
<tag>
code
goes
here
</tag>
more text
";

##</code><code>##

$string = "
hello
<tag><br>code<br>goes here<br></tag>
more text
";

##</code><code>##

$string =~ s/<tag>(.*?)<\/tag>/$1 =~ s/\n/<br>/gs;/ieg;