s{
    (\s*/[*].*?[*]/\s*)     # $1: /* comment */
  | \s*//[^\n]*             # // comment
  (                         # $2: something to keep:
  | '([^\\']+|\\.)*'        #   '\t'
  | "([^\\"]+|\\.)*"        #   "string"
  | /(?![/*])               #   Non-comment /
  | [^'"/]+                 #   Other code
  )
  | (.)                     # $3: A syntax error (unclosed ' or ")
}{
    if(  defined $3  ) {
        warn "Ignoring syntax error ($3) at byte ", pos(), $/;
    }
    $1          ? ' ' :     # "foo  /*...*/bar" => "foo bar"
    defined $2  ? $2  :     # Keep non-comment as-is
    defined $3  ? $3        # Keep syntax error as-is
                : ''        # "foo  // ...\n" => "foo\n"
}gsex;
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You just have to teach your regex to match things that might contain '/*' characters that don't represent comments. This mostly boils down to string literals. Though, if there is a chance of "// end-of-line" comments, then you have to match those as well. My code above strips them too.

(Updates made shortly after posting below:)

If you want to be defensive against mistakes in your regex or in your understanding of the syntax you are trying to parse, then you can add \G(?: and ) around the regex in order to prevent the possibility of it just skipping over unhandled stuff. You can then also specifically match "end of string" for similar reasons. I think the "(.)" case is simple enough that I have little worry of getting that part of the regex wrong and it serves the "misunderstood syntax" and "don't skip bits, including at end of string" purposes well enough.