use warnings;
use strict;

{ package Foo;
	our $DEFAULT_BAR = 1;
	sub foo {
		my %opts = @_;
		$opts{bar} //= $DEFAULT_BAR;
		print "bar=$opts{bar}\n";
	}
}

Foo::foo();          # prints "bar=1"
Foo::foo(bar=>2);    # prints "bar=2"