for my $param (@$mandatory) {
    ...
}

##</code><code>##

sub sort_params {
   return
       map $_->[0],
       sort { $a->[1] <=> $b->[1] }
       map [ $_, /(\d+)/ ],
       @_;
}

for my $param_key (sort_params keys %$mandatory) {
    my $param = $mandatory->{$param_key};
    ...
}