The thing to remember (and why concatenation worked) is that CGI scripts basically just output text. The text happens to be HTML text. And the CGI.pm "HTML shortcuts" are the functions that create the text. Since the HTML shortcuts' return values are all just strings, concatenation is an option in cases where you want two entities combined, and a comma would signify two separate items (such as in the case of lists or tables).

You kind of have to know how a given HTML shortcut looks in its text return-value form. For that, it's convenient to run the CGI script from the command line so you can see what output you're getting and why (or why not) it's doing what it's doing.