This is off-topic as it has nothing to do with Perl - but I'm feeling generous.

There are many ways to do it. Here are a couple:

• Set up your editor so that it automatically inserts a new datestamp in the file each time the file is saved. This is the default behaviour with html-mode in Emacs.
• If your server supports Server Side Includes then you can include the lines

  <!--#config timefmt="%D" -->
  This file last modified <!--#echo var="LAST_MODIFIED" -->
  [download]

  in your file. Of course you then need to ensure that your file is processed by the SSI processor. This is often as simple as giving the file a .shtml extension.

--
<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg