How can I dereference a hash in a subroutine an work on it like in the calling routine?

Question

I have a perl routine, that makes a hash out of a .csv-file. The values should be checked in a subroutine.

So I have a hash %my_values and call the subroutine check (\%my_values):

sub read_csv {
    ...

    # key:   headline entry
    # value: the value in the current row
    %my_hash;

    ...
    my ($has_error, $err_msg) = check (\%my_hash);
}

sub check {
    my($hash_ref) = @_;
    %my_hash = %$hash_ref;

    # Get the artikel number of the article
    $my_hash {'article_number'} = get_artnr($my_hash {'article'});
    if (not $my_hash{'article_number'}) {
        return 1, "Article $my_hash{'article'} not found!";
    }

    # check price (I'm in germany, there must be a conversation from ',' to '.')
    $my_hash {'price'} =~ s/,/./;
    if (not $my_hash{'price'} =~ m/^\d+(\.\d+)?$/) {
        return 1, "Invalid format of price";
    }

    return 0, "";
}

At first, this seems to work fine. But then i recognized, that neither the priceformat changed nor the key article_number is avalable.

Working directly on the reference made it:

# In this case, it works!
sub check {
    my($hash_ref) = @_;

    # Get the artikel number of the article
    $hash_ref->{'article_number'} = get_artnr($hash_ref->{'article'});
    if (not $hash_ref->{'article_number'}) {
        return 1, "Article $hash_ref->{'article'} not found!";
    }

    # check price (I'm in germany, there must be a conversation from ',' to '.')
    $hash_ref->{'price'} =~ s/,/./;
    if (not $hash_ref->{'price'} =~ m/^\d+(\.\d+)?$/) {
        return 1, "Invalid format of price";
    }

    return 0, "";
}

So i think %my_hash = %$hash_ref; makes a copy of the reference instead of dereferencing.

How can I dereference a hash in a subroutine an work on it like in the calling routine?

Answer 1

Here is an example using the new feature called refaliasing introduced in Perl 5.22 (optionally combined with the declared_refs feature introduced in 5.26)

use v5.26;
use warnings;  # IMPORTANT: this line must come before "use experimental"
use strict;
use feature qw(say); 
use experimental qw(declared_refs refaliasing);

{  # <-- introduce scope to avoid leaking lexical variables into subs below
    my %hash = (a=>1, b=>2);
    check(\%hash);
    say "Value of 'a' key is now: ", $hash{a};
}

sub check {
    my (\%hash) = @_;

    $hash{a} = 3;
}

Output:

Value of 'a' key is now: 3

Alternatively you can use the arrow operator:

sub check {
    my ($hash) = @_;

    $hash->{a} = 3;
}