For the low down on Perl references see perlref. You will probably find that perlsub helps too.

The important bits are:

The Perl model for function call and return values is simple: all functions are passed as parameters one single flat list of scalars

and:

Any arrays or hashes in these call and return lists will collapse, losing their identities

in:

my (...) = @_;
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the () provides a list context for the assignment so the elements in @_ are assigned in turn to the variables in (...). In the sample code there happens to be only one element in the list and we could instead have used one of:

my $hash = $_[0];
my $hash = shift;
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instead. If you omit the (), then the assignment sees a scalar and assigns the number of elements in @_ to $hash rather than the contents of the first element.

You would have to put a backslash in front of it normally, but because you've prototyped your sub Perl takes the reference for you behind the scenes and you receive that ref in $_[0] rather than the normal behavior of the hash being flattened into a list and you getting the key/value pairs in @_.

Of course one might feel inclined to yet again comment on the undesirability of prototypes, but I'm getting weary of doing so . . .

Thanks! I'll have to read up on prototypes. I barely follow the logic of why my program works at all at the moment.

Doug