The \b switch matches on word boundaries, \W matches on non word charachter, and \s matches whitespace (and of course you could match spaces. I would use \b (unless you want to do something else, because \s and \W will get substituted out), i.e.:

$sometext =~ s[\ba--\b][***]sgi;
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Also don't forget the i switch (so A-- doesn't go under the radar -- it does case insensitive)