With all due respect to jlistf, there were a couple of problems with his solution. Here's one that's easy to understand:

if ($filename =~ /^[A-Z]{5}/) {
    ($section_title = $filename) =~ s!:!:</B>\n<BR><BR>!;
    $filename = "";
}
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For $filename, we're just testing for the first five letters, so the {5,} syntax just makes the regex engine do more work.

For the actual substitution, the slash in </B> needs to be escaped unless we choose different delimiters like I have above.

Also, we can't do

$section_title = ($filename =~ s/?[:]./':</B>\n< BR>< B>\n');
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because the substitution returns a true/false value and if successful, $section_title receives a value of 1. The assignment to $section_title needs to be on a different line or assigned to as above (that's from a suggestion by KM).

Cheers,
Ovid

Update: Cleaned up the code in response to KM's point.