Easiest way to do it is to check how many parameters you're being passed, or work from the other end.

sub verify{
  my $optional;
  $optional = (@_ == 2) ? shift : "default value";
  my $ref = shift;
  ...
}
[download]

or

sub verify{
  my $ref = pop;
  my $optional = pop; #becomes undef if only one param
}
[download]