It seems likely that you aren't getting the data in the exact format you're expecting. Try adding some debugging lines:

while (my $input = <$sock>) {
  print "input line: **start**$input**end**\n";  # debugging
        if (index($input, "/QUOTE") != -1) {
  print "    is a match\n";  # debugging
           my $str = $input;
           ($pass) = $str  =~ m{/QUOTE\sPASS\s(.*?)$};
           print $sock "QUOTE PASS $pass\r\n";
        } else {
  print "    not a match\n"  # debugging
        }
}
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One thing I notice is that you're both reading from and writing to the filehandle in $sock. That might be possible if that's a particular kind of object, but it definitely won't work if it's a normal pipe or filehandle, so it might be a mistake.