I'm not sure if I understand your problem ...

... the call foo("a") is exactly doing what I expect, i.e. @_ == 1

Anything else - like pushing an optional $that into @_ - wouldn't really be backwards compatible.

And the number you seem to want is static, it's the sum of obligatory arguments + defaulted ones.

(maybe you want to make a feature request to be able to introspect the signature)

FWIW, when interested you can look into the implementation with B::Deparse ...

# 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 @_;
}


is(
   foo(1),
   1,
   "one arg"
  );

is(
   foo(1,2),
   2,
   "two args"
  );


done_testing();


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

C:/Perl_524/bin\perl.exe -w d:/exp/pm_signatures.pl 
ok 1 - one arg
ok 2 - two args
1..2
Show implementation:
{
    BEGIN {${^WARNING_BITS} = "\x54\x55\x55\x55\x55\x55\x55\x55\x55\x5
+5\x55\x55\x55\x55\x54\x55\x05"}
    use strict;
    use feature 'signatures';
    die sprintf("Too many arguments for subroutine at %s line %d.\n", 
+(caller)[1, 2]) unless @_ <= 2;
    die sprintf("Too few arguments for subroutine at %s line %d.\n", (
+caller)[1, 2]) unless @_ >= 1;
    my $this = $_[0];
    my $that = @_ >= 2 ? $_[1] : 'bar';
    ();
    return scalar @_;
} at d:/exp/pm_signatures.pl line 32.

Compilation finished at Tue Jul 14 14:25:41
[download]

UPDATE

expanded code with tests