First, note that Perl is case-sensitive, and the pseudo-package (not keyword) is SUPER. The short answer is that it allows a derived class to call its parent's version of an overridden method without hard-coding the parent package in the method, i.e.:

sub get_coordinates {
    $self = shift;
    return $self->SUPER::get_coordinates(), get_z();
}
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This is explained in much more detail in the Overridden Methods section of perltoot, which you should read if you want to learn about OO mechanics in Perl.