with perl 5.8.4 on linux(debian unstable), the following code

      1 #!/usr/bin/perl
      2 
      3 use strict;
      4 
      5 my $string = 'test("test ... bla bla bla")';
      6 $string =~ s/test\("test/test\("GC_test/;
      7 
      8 print $string."\n";
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return test("GC_test ... bla bla bla")

perhaps the probleme is your verion of perl