$source is a reference to a hash. You find the keys by de-referencing it (%$source) then you dereference it with the arrow in $source->{node}. You could use the format you are used to if you do something like:
my %newSource = %$source; # dereference it all at once
foreach my $node(keys (%newSource)) { # note no %$
#...
$newSource{$node}; # already dereferenced.
update I'm not recommending that you change the code, but showing a similar situation to explain the code. If you are thinking of changing the code, see the important caveat mentioned by Random_Walk.