You can pass a hash as parameters list:

sub prog {
  my %args =
    ( a => 'default_a',
      b => 'default_b',
      c => 'default_c',
      d => 'default_d',
      @_
    );
  print "a => $args{a}\n";
  print "b => $args{b}\n";
  print "c => $args{c}\n";
  print "d => $args{d}\n";
}
prog( b => 1, a => 2);
[download]