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 ~=

This will not answer your question but:

I'm pretty sure you meant =~ and not ~=

Sometimes I forget if it's =~ or ~=, so I use the following mnemonic: it is NOT supposed to be a contraction for $var = $var ~ $some_regex, so it is NOT ~=.

Yup, looks like roll-your-own-template cleverness.   If you can get rid of it, do so.   And if you can’t, document the hell out of it ...