Passing only some subroutine arguments by reference in Perl - perl

Question

I'm writing a subroutine that takes a number of arguments. Most of those arguments are the standard pass-by-value sort, where changes made to them within the subroutine don't matter outside of it. But one of them is an object (blessed reference) that I'd like to make changes to that are available outside of the subroutine, if it's passed in. If it's not passed in, I would like to instantiate it and treat it the same way as if it were passed in (but returning it in the end).
For example:
my $foo = Private::Foo->new();
# $foo->{'something'} eq 'old value'
Private::Foo->do_things('abc', 'xyz', $foo);
# $foo->{'something'} eq 'new value'
my $foo2 = Private::Foo->do_things('def');
# $foo2->{'something'} eq 'old value'
package Private::Foo;
# ...
sub do_things {
my ($self, $arg1, $arg2, $foo) = #_;
unless (defined $foo) {
$foo = Private::Foo->new();
}
if ($arg1 eq 'abc') {
$foo->{'something'} = 'new value';
return;
}
return $foo;
}
I'd like to do this with as clean of syntax as possible, and I'm fine using any features available in Perl v5.22 and higher. (I've tried to figure out how to do this using refaliasing, but it isn't very clean.)
What am I missing?

Answer 1

First of all, subroutine arguments are always passed by reference.
$ perl -e'sub f { $_[0] = "def"; } my $x = "abc"; f($x); CORE::say $x;'
def
More importantly, your code does exactly what you asked already.
$ perl -e'
{
package Private::Foo;
sub new { my $class = shift; bless({ something => "old_value" }, $class) }
sub do_things {
my ($self, $arg1, $arg2, $foo) = #_;
unless (defined $foo) {
$foo = Private::Foo->new();
}
if ($arg1 eq "abc") {
$foo->{something} = "new value";
return;
}
return $foo;
}
}
use feature qw( say );
my $foo = Private::Foo->new();
Private::Foo->do_things("abc", "xyz", $foo);
say $foo->{something};
my $foo2 = Private::Foo->do_things("def");
say $foo2->{something};
'
new value
old_value
That said, you could clean up your method some:
sub do_things {
my ($class, $arg1, $arg2, $foo) = #_;
$foo //= $class->new();
if ($arg1 eq 'abc') {
$foo->{something} = 'new value';
}
return $foo;
}
It would be even better if you cleaned up your calling convention.
Private::Foo->do_something($arg1, $arg2, $foo);
my $foo2 = Private::Foo->do_something($arg1, $arg2);
makes far less sense than
$foo->do_something($arg1, $arg2);
( my $foo2 = Private::Foo->new )->do_something($arg1, $arg2);