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I'm looking at using Attribute::Handlers as the basis for a debugging output system in some code I'm writing. So far, I have something like this: 
use Attribute::Handlers;
use vars qw($DEBUG);

$DEBUG = 0;

use constant {
    PKG     => 0,
    SYMBOL  => 1,
    CODE    => 2,
    ATTR    => 3,
    DATA    => 4,
    PHASE   => 5,
};

sub Debug :ATTR {
    my ($symbol, $code, $level) = @_[SYMBOL, CODE, DATA];
    $level = $level ne '' ? $level : $DEBUG ;
    my $name = join '::', *{$symbol}{PACKAGE}, *{$symbol}{NAME};
    no warnings 'redefine';
    *{$symbol} = sub {
        my @output = $code->(@_);
        if ( $level >= 2 ) {
            warn sprintf "DEBUG: entering %s(%s)\n",
                $name, ($level >= 3 ? "@_" : '');
        }
        return @output;
    };
}

sub somesub :Debug {
  # Do something.
}

sub othersub :Debug(3) {
  # Do some other thing.
}

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One part of this feels inelegant to me, namely: 
$level = $level ne '' ? $level : $DEBUG ;

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It has been a while since I was heavily into Perl, so I'm wondering if I'm just forgetting some prettier way to do it... but it keeps nagging at me every time I look at that chunk of code. Anybody know a nicer way to assign a value to a variable only if it is currently NULL? 
$level ||= $DEBUG;

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doesn't work, presumably because a null string evaluates to a true value. But that has the flavor I'm looking for.