Meta Information, caller(), ref(), isa(), ...

($package, $file, $line) = caller();
Call stack information.

foreach $k (keys %Foo::) ...;
The global symbols of the package Foo.

print ref($r);
What a reference contains.

$obj->isa("Foo");
Returns true, if $obj inherits Foo.

$obj->can("bar");
Returns true, if it supports method bar.