But, I'm not able to see that cookie when I print $ENV{'HTTP_COOKIE'}
within in header_parse_handler.
%ENV is setup during the fixup phase, so it hasn't been populated yet.
yes, using something like $r->headers_in. you're actually better off using Apache::Cookie, which is in the libapreq distribution on cpan.mod_perl docs say that that you can examine request headers in the PerlHeaderParserHandler.
if you want to force %ENV to be setup earlier, try calling
$r->subprocess_env;
in a void context before checking %ENV - it used to segfault for me, but the docs says it should work.
HTH
--Geoff