Another way to do this is with a separate %DEFAULTS hash that gets mixed in with the arguments passed.

use Data::Dumper;

my %DEFAULTS = ( a => 'a default',
                 b => 'b default',
                 c => 'c default', );
my %args = ( a => 'a arg',
             b => undef, );
%args = ( %DEFAULTS, %args );

print Dumper \%args;
__END__
$VAR1 = {
          'c' => 'c default',
          'a' => 'a arg',
          'b' => undef
        };
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Notice that this method keeps a value that exists but is not defined, so somebody can explicitly pass in undef if they want to. This may or may not be what you want.