Help understand documentation

This is from http://perldoc.perl.org/Net/Ping.html

# Like tcp protocol, but with many hosts
$p = Net::Ping->new("syn");
$p->{port_num} = getservbyname("http", "tcp");
foreach $host (@host_array) {
$p->ping($host);
}
while (($host,$rtt,$ip) = $p->ack) {
print "HOST: $host [$ip] ACKed in $rtt seconds.\n";
}

Can some body tell me what the following line does?
$p->{port_num} = getservbyname("http", "tcp");

--
Vijay
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