Perl auto-flattens these => to a comma, so instead of a single hash as parameter, it is converted automatically to an array of many parameters.

You can avoid it, but it is clunky:

use 5.010;

sub send_3{
  my ($h) = @_;

  # 'fix' if broken parameters
  unless(ref($h) eq 'HASH'){
    my %X = @_;
    $h = \%X;
  }

  # set default values
  $h->{foo} //='default';

  # iterate over parameters
  for my $key (keys %$h){
    say "$key ~> $h->{$key}"
  }
}


send_3( {"foo"=>42, "bar"=>'code'});
send_3( "baz"=>11, "zab"=>'dong');
[download]

Which yields:

fbrm@monastery:~/CODE/PERL/monks$ perl x
foo ~> 42
bar ~> code
zab ~> dong
foo ~> default
baz ~> 11
[download]

Note that starting from perl 5.10, you can use the "//=" operator to test if the variable is defined, and not just non-zero. (Imagine the call send_3(foo=>"0", bar=>"cool"), which using the above would yield "default cool", which might not be what you wanted. Using the //= operator it would yield "0 cool").