Question

The purpose is to only ammend the hashref argument being passed to the function, but not modify the original hashref.

{
    my $hr = { foo => q{bunnyfoofoo},
               bar => q{barbiebarbar} };

    foo( { %$hr, baz => q{bazkethound} } );
}

sub foo {
   my $args = shift // {};
}

I think the above would work, but am not sure there is a better way (other than modifying the orginal ref before passing).

Answer 1

Your code is as simple as it can be. You need to pass a hash and you don’t want to modify the existing hash, so you need to create a new hash. That’s what { } does.

The underlying problem is foo takes a hash ref instead of a key-value list.

sub foo {
   my %args = @_;
   ...
}

You can still do everything you did before:

foo( foo => 'bunnyfoofoo', bar => 'barbiebarbar' );

my $hash = {
   foo => 'bunnyfoofoo',
   bar => 'barbiebarbar',
};
foo( %$hash, baz => 'bazkethound' );

There are two advantages.

1. You don’t have to use { } all over the place.

2. You can modify the argument safely.

   This is buggy:

   sub f {
      my $args = $_[0] // {};
      my $foo = delete($args->{foo});
      my $bar = delete($args->{bar});
      croak("Unrecognized args " . join(', ', keys(%$args)))
         if %$args;
      ...
   }
   

   This is ok:

   sub f {
      my %args = @_;
      my $foo = delete($args{foo});
      my $bar = delete($args{bar});
      croak("Unrecognized args " . join(', ', keys(%args)))
         if %args;
      ...
   }