G'day Dr Manhattan,

You have a number of problems here.

When you call probability(), @_ contains all the keys and values from %hash, the hashref $hash{Mary}, and the value 0.8 from $hash{Mary}{Maria} (which is actually the value you want returned, so calling a subroutine is pointless in this instance).

When assigning those values to (%hash, $source, $target), they are all assigned to %hash leaving $source and $target undefined.

You're calling probability() with a leading ampersand (&). This probably isn't what you want and may (in the context of your real progam) be doing something unexpected. Don't add leading ampersands to your subroutine calls unless that's really what you want and you understand the implications of what you are doing! See perlsub for a more detailed discussion.

The code I suspect you want is probably closer to this:

$ perl -Mstrict -Mwarnings -e '
    my %hash = (Mary => { Maria => 0.8}, other_keys => "other_values")
+;

    sub probability
    {
        my ($hash_ref, $source, $target) = @_;
        return $hash_ref->{$source}{$target};
    }

    my $test = probability(\%hash, "Mary", "Maria");
    print "$test\n";
'
0.8
[download]

Another possibility is:

my %hash = (Mary => { Maria => 0.8}, other_keys => "other_values");

sub probability (\%$$)
{
    my ($hash_ref, $source, $target) = @_;
    return $hash_ref->{$source}{$target};
}

my $test = probability(%hash, "Mary", "Maria");
print "$test\n";
[download]

package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

As it has already been said, it seems pointless to use a subroutine in such a case. However, assuming you want to fix your code and understand what's going on, you would need to pass a hash ref, rather than the hash itself, as explained by others above, including kcott. Or another fix would be to put your hash as the last element of your argument list:

sub probability
    {
        my ($source, $target, %hash) = @_;
        return $hash{$source}{$target};
    }
my $proba = probability($key_level1, $key_level2, %hash);
[download]

This way, the hash does not "eat" the trailing arguments.

Update: Corrected a copy_and_paste error and the missing name of the subroutine in the function call. I had to go, I was too hasty on typing this.

You are not passing a hash ref. and neither do you have the variable $hash_ref defined. Then the subroutine is not called anywhere.

Can't be done, you can only send references

Basic debugging checklist item 4 ( Dumper )

my( %hashgetsthemall, $two, $eleven ) = 1 .. 11;
dd \%hashgetsthemall, $two, $eleven ;
__END__
(
  { 1 => 2, 3 => 4, 5 => 6, 7 => 8, 9 => 10, 11 => undef },
  undef,
  undef,
)
[download]

my( %hashgetsthemall, $two, $eleven ) = 1 .. 11;

my( $one, $two, %hashgetstherest ) = 1 .. 12;

is one way to do it if you wanted copies of the values anyway.
The list variable must come last though, and there can be only one.

update:

In the OP's example that is:

my ($source, $target, %hash) = @_;

Cheers, Sören

Créateur des bugs mobiles - let loose once, run everywhere.
(hooked on the Perl Programming language)

Other form of greediness in Perl discusses how assignment to @array and %hash is greedy, they're sinks, they eat everything