header() looks a lot like a method (or exported sub) found in the CGI.pm module. And the fact that you're using CGI::Carp does somewhat lend support for the notion that this is a CGI script. So if you're going to use the CGI module's subroutines, you should probably "use CGI qw/:standard/;"

While you're at it, be sure to read the POD for CGI.