When you pass an argument to a function, @_ is an alias to these arguments.

So the following works: 

use strict;
my $x = 1;

modify_args($x),
print $x, "\n";

sub modify_args {
    $_[0] = "modified";
}
__END__
modified

[download]
But as soon as you assign the values to a new variable, you create a copy: 
my $x = 1;

modify_args($x), # doesn't work
print $x, $/;

sub modify_args {
    my ($copy) = @_;
    $copy = "modified";
}
__END__
1

[download]

Which means that you can change any arguments passed to a sub as long as you stick to manipulating them in @_.

You can circumvent this restriction by passing references , using a module like Data::Bind or Data::Alias), or by creating a new hash and returning that.

If you don't like the last solution (returning a new hash) you should stick to the first one, because of the principle of least surprise: When you call a sub like this: sub_name %hash you don't expect %hash to be modified.

Another possible options are prototypes (notable (\%)), but they come with many problems, and you shouldn't use them until you understood all of these problems

In reply to Re: Modify a hash via its reference by moritz
in thread Modify a hash via its reference by Philippe

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