in reply to Do I understand this?

Yes, you do!

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'

Cheers Rolf

UPDATE: corrected C&P typo ~=