quoted from: http://search.cpan.org/src/LDS/CGI.pm-3.25/cgi_docs.html
(stale; no updating since Feb 2005, but perhaps helpful anyway:

To add attributes to an HTML tag, simply pass a reference to an associ
+ative array as the first argument. The keys and values of the associa
+tive array become the names and values of the attributes. For example
+, here's how to generate an <A> anchor link:

   use CGI qw/:standard/;
   print a({-href=>"bad_art.html"},"Jump to the silly exhibit");

   <A HREF="bad_art.html">Jump to the silly exhibit</A>

You may dispense with the dashes in front of the attribute names if yo
+u prefer:

   print img {src=>'fred.gif',align=>'LEFT'};

   <IMG ALIGN="LEFT" SRC="fred.gif">

Sometimes an HTML tag attribute has no argument. For example, ordered 
+lists can be marked as COMPACT, or you wish to specify that a table h
+as a border with <TABLE BORDER>. The syntax for this is an argument t
+hat that points to an undef string:

   print ol({compact=>undef},li('one'),li('two'),li('three'));

Prior to CGI.pm version 2.41, providing an empty ('') string as an att
+ribute argument was the same as providing undef. However, this has ch
+anged in order to accomodate those who want to create tags of the for
+m <IMG ALT="">. The difference is shown in this table:
[download]

CODE	RESULT
img({alt=>undef})
img({alt=>''})	<IMT ALT="">