sub your_function {
     my ( $string, $in_fh, $out ) = @_;
     local *OUTFH;

     # opens $out if it's a filename
     # dupes $out if it's a filehandle
     open OUTFH, ">$out" or die "$out: $!\n";

     # do whatever with $in_fh and $string
     while( <$in_fh> ) {
       /$string/o and print OUTFH "$string: $_"
     }

     # close OUTFH: either we opened it or we duped it
     # Either way we don't need it any more.
     close OUTFH;
   }