tie( $line, 'Queue', 'a b c d' ); calls the TIESCALAR sub. First arguement my $class = shift; is 'Queue'. 'a b c d' goes into my @data  = split ' ', $_[0];.

Whenever you "read" $line (which includes print) FETCH is called. Object reference will be passed to my $obj = shift;. @$obj in FETCH is the dereferencing of $line pointing to \@data in TIESCALAR.

STORE kind of overloads the operator =.