in reply to Do I understand this?
It deletes HTML-comments, even if they span multiple lines.
Anyway it's better (and less buggy) written as $var =~ s/<!--.*?-->//gs
EDIT: Please note the .*? for non-greedy now, otherwise you are deleting everything in between the first and the last HTML-comment.
see also Regex: Char-class equivalent for [^] for a recent discussion of '.' and '/s'
UPDATE: corrected C&P typo ~=