my $post_body ='<c>this is the first code block code</c> This is <c>another code block</c>';
my $part = 1;
$part++  while $post_body =~ s/<c>(.*?)<\/c>/<a href='download?part=$part'>Download Me<\/a><pre>$1<\/pre>/i;
print $post_body, "\n";

##</code><code>##

<a href='download?part=1'>Download Me</a><pre>this is code</pre> This is <a href='download?part=2'>Download Me</a><pre>more code</pre>