See documentation for LWP::UserAgent get method

use LWP::Simple qw/ $ua /;
$ua->timeout(0.1);
$ua->add_handler("request_send",  sub { shift->dump; return });
$ua->get( 'http://localhost' );
$ua->get( 'http://localhost', 'X-Requested-With' => 'XMLHttpRequest');
__END__
GET http://localhost
User-Agent: LWP::Simple/6.00 libwww-perl/6.05

(no content)
GET http://localhost
User-Agent: LWP::Simple/6.00 libwww-perl/6.05
X-Requested-With: XMLHttpRequest

(no content)
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See also LWP::Debug