Postulate: because @_ is a magic variable. Witness two different approaches which worked for me:

sub foo {
    my $self = shift;
    my @args = @_;
    $self->{bar} = ref ( $_[0] ) ? $_[0] : \@args;
}
[download]

(my preferred approach)

sub foo {
    my $self = shift;
   $self->{bar} = ref ( $_[0] ) ? $_[0] : [ @_ ];
}
[download]

My suspicion is that @_ gets clobbered (or unset) as soon as you leave the subroutine due to internal Special Magic. You're left holding a reference to an empty variable. Either copy the values out of it, or expand it into an anonymous list.