# call the sub
my_sub(userid => 'foo', password => 'bar');

sub my_sub {
  my %default_values = (userid => 'user', password => 'passwd');

  my %param = (%default_values, @_);

  print $param{userid}, $/;
  print $param{password}, $/;
  # ...
}

# or with all required arguments
sub my_sub {
  my @required = qw/userid password/;
  my %param = @_;

  for my $req (@required) {
    die "Subroutine my_sub called without required parameter: $req"
      unless exists $param{$req};
  }

  print $param{userid}, $/;
  print $param{password}, $/;
  # ...
}