To allow defaults and/or enforce mandatory values in a hashref:

my %args = (
    key1    =>  'specified 1',
    key3    =>  'THIS SPACE FOR RENT'
);

my $hr = {
    key1    =>  'default 1',
    key2    =>  'default 2',

    %args,

    key3    =>  'enforced 3'
};

## now:
##  $hr->{key1} == 'specified 1'
##  $hr->{key2} == 'default 2'
##  $hr->{key3} == 'enforced 3'
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In English: when building a hash, key => value pairs override the previous value for the given key. And tossing a %hash (or @array, if it looks like a hash) in the mix will just expand itself to a list of key, value, key, value...