I assume that you're interested in func1 followed by balanced parenthesis so this might be of use.

#!/usr/bin/env perl

use strict;
use warnings;

my $balpar_re=qr/
      (
        \(                                 # opening (
          (?:[^'"()]++                     # Not a ', ", ( or ) - no b
+acktracking
            |(?:'(?:[^']|\')*?')           #' a single quote string
            |(?:"(?:[^"]|\")*?")           #" a double quote string
            |(?-1)                         # must be a ( or ) so recur
+se
          )*+                              # zero or more time - no ba
+cktracking
        \)                                 # 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__
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