Hmmm I was playing with the comment sub expression and I think this might work (dunno, i played with look aheads but those arn't realy my speciality. ;))

my $test = qr/(<!--(?:[-][^-]|[^-])*-->)/;

for ( "<!-- -->", "<!----->", "<!--- -->", "<!---->" , "<!-- - -- -->"
+) {
    print $_, ($_ =~ $test ? "ok": "fail"),"\n";
}
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So it should match anything that looks like a comment and the only tuime it has two dashes in a row is at the begging and at the end. Needs mroe vigerous testing though.