use Regexp::Common qw/balanced/;

while (<DATA>) {
   chomp;

   my $bp = qr/$RE{balanced}{-parens=>'()'}{-keep}/;

   $_ =~ s{
      &?                # optional &
      (                 # capture either:
         (?<=&) \w+     #    bareword preceeded by &
      |                 # or
         \w+ (?=\s*\()  #    bareword followed by (
      )                 #
	  (?: \s* (?=\())?  # optional white space, if followed by (
      (?:               # either:
         $bp            #    balanced parens
      |                 # or
                        #    nothing!
      )
   } 

   # if no second capture, supply an empty () 
   { $1 . ($2 || '()') }exg;

   print "$_\n";
}

__DATA__
&foobar
&foobar()
foobar()
&foobar("asdf")
&foobar( test(), qw(etc.))
&foobar( test(test() . test()) )
&foobar("a", 5, qw(dave jen anne matt), map({tr/[a-z]/[A-Z]/; $_;} @mixedCase));
&dont_change if not &function()
&foobar ('<- white space?')
&foobar( &test_nested )