Question

I’m sending a subroutine a hash, and fetching it with my($arg_ref) = @_;

But what exactly is %$arg_ref? Is %$ dereferencing the hash?

Answer 1

$arg_ref is a scalar since it uses the $ sigil. Presumably, it holds a hash reference. So yes, %$arg_ref deferences that hash reference. Another way to write it is %{$arg_ref}. This makes the intent of the code a bit more clear, though more verbose.

To quote from perldata(1):

 Scalar values are always named with '$', even when referring  to a scalar that is part of an array or a hash.  The '$'  symbol works semantically like the English word 'the' in  that it indicates a single value is expected.       $days               # the simple scalar value 'days'      $days[28]           # the 29th element of array @days      $days{'Feb'}        # the 'Feb' value from hash %days      $#days              # the last index of array @days 

So your example would be:

     %$arg_ref           # hash dereferenced from the value 'arg_ref' 

my($arg_ref) = @_; grabs the first item in the function’s argument stack and places it in a local variable called $arg_ref. The caller is responsible for passing a hash reference. A more canonical way to write that is:

my $arg_ref = shift; 

To create a hash reference you could start with a hash:

some_sub(\%hash); 

Or you can create it with an anonymous hash reference:

some_sub({pi => 3.14, C => 4}); # Pi is a gross approximation.