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.
}

##</code><code>##

$level = $level ne '' ? $level : $DEBUG ;

##</code><code>##

$level ||= $DEBUG;