It seems to me not a question about perl at all. That's not a problem per se but a missing answer for 5 hours might indicate that perlmonks just don't know

You need to find out how to specify transparent background in CSS (and possibly how to selectively apply that to parts of a website, if you need that feature). Read the CSS documentation, look into CSS tutorials or books, or simply ask in a forum full of HTML designers. Type 'css forum' into google and just pick one.

Then just put the correct css as inline css into the script somewhere near line 20:

...
print "HTTP/1.0 200 OK\r\n", $cgi->header, $cgi->start_html("Tag Cloud
+");

    #now the new line
    print qq{<style type="text/css">\n} . "***here is your css***" . "
+</style>";

print $main::cloud->html_and_css();
...
[download]

PS: I shamelessly copied the new line from HTML::TagCloud, where some minimal css is generated in the sub css(), which is called from html_and_css(), which is called from the last line of the code above