When you call m/regex/g in scalar context, it first returns the first occurrance of regex. If you call it again, it'll return the second occurance of regex, and so on.

In using if, you're only calling the regexp once per line. Change it to this:

while (<file>) {
  while (/(regex1|regex2|regex3)/gi) {
   f($1);
  }
}
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Update: Added the 'g' modifier. Thanks to Hofmator for keen eyes.

stephen