# Unfortunate scalar conversion
my @foo = @_ || @bar;

# Unfortunate error, not DWIM-compatible
my @foo = @_;
@foo ||= @bar;

##</code><code>##

my @bar = qw[ dog cat frog ];

sub foo 
{
        my @foo = grep{length}(@_, (!@_ && @bar));

        print join(',', @foo),"\n";
}