Disappointments:

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

# Unfortunate error, not DWIM-compatible
my @foo = @_;
@foo ||= @bar;
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Although there are many good solutions posted here, how about a bad one?

my @bar = qw[ dog cat frog ];

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

        print join(',', @foo),"\n";
}
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Hey, it works, but it's quite brittle. If you even think about changing the order of the parameters, you're into SCALAR land.