# https://perlmonks.org/?node_id=11119273
use strict;
use warnings;
use feature qw(signatures);
no warnings qw(experimental::signatures);

use B::Deparse;
use Test::More;


sub foo ($this, $that='bar') {
    return scalar @_;
}

t_foo(0,undef,'Too few arguments');
t_foo(1,1,"One arg");
t_foo(2,2,"Two args");
t_foo(3,undef,"Too many arguments");

done_testing();

sub t_foo {
    my ($n_args, $exp, $desc ) = @_;

    my @args = ("","a".."z")[1..$n_args];
    my $code = "foo(qw/@args/)";

    my $got = eval $code;
    unless ($@) {
        is( $got, $exp,
            "<$code> \t $desc: \@_ == $got"
          );
    } else {
        ok( $@ =~ $desc,
            "<$code> \t ERR: $desc: ");
    }
}

#diag "Show sig implementation:\n", B::Deparse->new()->coderef2text(\&
+foo);
[download]

C:/Perl_524/bin\perl.exe -w d:/exp/pm_signatures.pl 
ok 1 - <foo(qw//)>      ERR: Too few arguments: 
ok 2 - <foo(qw/a/)>      One arg: @_ == 1
ok 3 - <foo(qw/a b/)>      Two args: @_ == 2
ok 4 - <foo(qw/a b c/)>      ERR: Too many arguments: 
1..4
[download]