$source is a reference to a hash. You find the keys by de-referencing it (%$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.

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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.

In reply to Re: The Drivel is in the Details by jZed
in thread The Drivel is in the Details by dalgetty

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