#!/usr/bin/env perl

use strict;
use warnings;

my $balpar_re=qr/
      (
        \(                                 # opening (
          (?:[^'"()]++                     # Not a ', ", ( or ) - no backtracking
            |(?:'(?:[^']|\')*?')           #' a single quote string
            |(?:"(?:[^"]|\")*?")           #" a double quote string
            |(?-1)                         # must be a ( or ) so recurse
          )*+                              # zero or more time - no backtracking
        \)                                 # closing )
      )
     /x;

my $text=<<'__EOT__';
   xfunc1("shouldn't be changed!") 
   func1 (abc,"abc","should,be,changed!");
   func1(arg(),narg());
__EOT__

if ($text=~ s{\bfunc1\s*$balpar_re}{newtext}g) {
    print $text;
    };
__DATA__