What about named parameters?

sub doit {
   my %options = @_;
   if ($options{filename} ) {
       #use a filename
   }
   elsif ( $options{filehandle} ) {
      # use a filehandle
   }
   else {
      die "won't work";
   }
}

doit( filename => 'somefile');
doit( filehandle => $fh );
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