Don't get me wrong - TIMTOWTDI. But how is that expression nearly as clear as index($str,  $whatever) > -1? For the system, it is probably equivalent regarding the overall number of operations. For the human brain (mine at least), it is one less.

Select wisely the idioms you use. Others reading your code will thank you. I, for one, would not use that one because I find it is just for the sake of brevity without its being worth one more thought every time it is read.

while ( ($p = index $haystack, $needle, $p+1) > -1 ) {
    # e.g. (not tested)
    pos($haystack) = $p;
    $p = pos($haystack) if $str =~ s{ \G ... }{...}cxms
        # regex doing the tough dishes
    ;   # /c flag: preserve pos() on non-match
}
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