#!/usr/bin/perl -w

use strict;
my $str_in = 'test("test';
my $str_out = $str_in;
#$str_out =~ s/"t/"GC_t/;
$str_out =~ s/"/"GC_/;
print "string 1: $str_in\n";
print "string 2: $str_out\n";
[download]

produces

string 1: test("test
string 2: test("GC_test
[download]

Am I missing something here?

Update: Just match the double quote (original match commented out above). That works just well, assuming there is only one double quote in the string.