Name "main::OUTPUT" used only once: possible typo at ... .
missing E:/qdepot_automation/addfiles.txt

Unrecognized escape \q passed through in regex; marked by <-- HERE in m/E:\q <-- HERE depot_automation/ at ... .
missing E:/qdepot_automation/AMSS/products/build/textfiles.txt

##</code><code>##

use strict;
use warnings;
my $client_root = "E:/qdepot_automation";

my $perforce_root_dir = "//depot/code";
my $line;

while ($line = <DATA>) {
    if ($line =~ /missing/) {
        $line =~ s/\\/\//g;
        $line =~ s/\Q$client_root\E/$perforce_root_dir/g;
        print "$line\n";
    }
}

__DATA__
missing E:\qdepot_automation\addfiles.txt
same E:\qdepot_automation\AMSS\products\build\ms\files.data
same E:\qdepot_automation\AMSS\products\build\ms\build.cmd
missing E:\qdepot_automation\AMSS\products\build\textfiles.txt
same E:\qdepot_automation\AMSS\products\build\ms\lib.min

##</code><code>##

missing //depot/code/addfiles.txt

missing //depot/code/AMSS/products/build/textfiles.txt