In Perl Best Practices, TheDamian recommends using a hash for default parameters like so: 

my %DEFAULTS = ( a => 1, b => 2 );
sub foo {
    my $arg_ref = shift;
    my %args = ref $arg_ref eq ref {} ? ( %DEFAULTS, %{$arg_ref} )
                                      : %DEFAULTS;
}

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He clearly expects the second hash to overwrite the duplicated contents of the first, so I think it's safe to say this is defined behavior that you can rely on.

In reply to Re: Assigning list with duplicate keys to hash by kyle
in thread Assigning list with duplicate keys to hash by mscharrer

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