Use experimental Perl feature:

  #!/usr/bin/perl
  use Modern::Perl '2015';
  use feature qw(signatures);
  no warnings qw(experimental::signatures);
  
  sub foo($bar, $baz) { $bar+$baz }
  
  say foo(40, 2);

See `man feature` for details.

Or use CPAN modules: https://metacpan.org/pod/signatures , https://metacpan.org/pod/Function::Parameters