my $braces = qr{
		   \(
		   (?:
			  (?> [^\(\)"]+ )    # Non-parens without backtracking
			|
			  (?> "[^"]*")+		# skip string constants
			|
			  (??{ $braces })     # Group with matching parens
		   )*
		   \)
		}x;
my $Braces = qr{\(((?:[^\(\)"]+|"[^"]*")*(?:$braces(?:[^\(\)"]+|"[^"]*")*)*)\)};

my $str = 'foo( FieldName, trim(CStr(Request.Form(FieldName))), CountryId )';

$str =~ $Braces and print "YES: $1\n" or print "NO!\n";