I'd suggest using:

$pattern = "whatever";
$mode = "(?";
$mode .= "i" if $case_insensitive;
$mode .= "s" if $dot_matches_all;
$mode .= "m" if $multiline_match;
$mode .= "x" if $allow_comments;
$mode .= ")";
$count++ while $string =~ /$mode$pattern/o;
[download]

Voila.