You have a choice. Firstly, you can send a plain text attachment that contains the URL. As doward explained, most modern mail programs will recognise the the URL and turn it into a clickable link when the mail is displayed. In this case, your mail looks like this:

my $message_body = <<END_OF_BODY;
This is a list of things to bring
* Fresh flowers from Flowershop X
    http://www.example.com/flowers

* Today's Arrangement instructions from instruction home page
    http://www.example.com/arrangement/
END_OF_BODY
[download]

That's the only way it can work in plain text. You can just include the URL and rely on the mail program to make it clickable.

Alternatively, you can create a HTML version of the mail which contains real <a> tags. In this case, your mail looks something like this:

my $message_body = <<END_OF_BODY;
<p>This is a list of things to bring</p>
<ul>
<li>Fresh flowers from <a href="http://www.example.com/flowers/">Flowe
+rshop X</a></li>
<li>Today's Arrangement instructions from <a href="http://www.example.
+com/arrangement/">instruction home page</a></li>
</ul>
END_OF_BODY
[download]

In this second example, you would need to set the content type of the attachment as 'text/html' so that the mail program that is displaying it knows how to treat it.

As several people have mentioned in this thread, if you want HTML mail, then the best approach is to have two attachments, one of which is an HTML version and the other of which is the plain text version. In this case, you need to find out how your Mail-building module of choice (and I can only repeat once more my recommendation for the Perl Email Project) builds MIME messages with more than one attachment.