Xyzzy::say_it;
# Calls sub say_it in package Xyzzy with an empty parameter list

Xyzzy->say_it;
# Calls sub say_it in package Xyzzy with the parameters ('Xyzzy')

my $obj = Some::Class->new(foo => 'bar');
# Calls sub new in package Some::Class with the parameters 
# ('Some::Class', 'foo', 'bar')
#
# By convention, 'new' will bless its return value into the class
# (package) matching its first parameter ('Some::Class'), making
# $obj an instance of that class

$obj->do_stuff(1, 2, 3);
# Because $obj is blessed as an instance of Some::Class, calls
# Some::Class::do_stuff with the parameters ($obj, 1, 2, 3)