I really like this answer. It prompted me to think more widely about the issue, looking at functions (built-ins and CORE modules) that might be helpful. This led to the construction that I'm now favoring: 

sub f { # ( [$:arg] )
    use 5.007003; # perl v5.7.3; required for List::Util to be availab
+le in CORE
    use List::Util qw/ first /;
    my $arg = first { defined && length } ( shift, 'default_value' );
    $arg;
    }

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In reply to Re^2: Assigning default values to function arguments which may be “empty” by Wyrdweaver
in thread Assigning default values to function arguments which may be “empty” by Wyrdweaver

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