Correct you are, my bad. I'm used to calling that in a different context so I looked back at the code I had actually been using. This is what would actually work the way I described:

use strict;
use warnings;

my $c = 1;
my $d = undef;

sub sbr {

    $c = 2 unless $c;
    print "c = $c\n";

    $d = 4 unless $d;
    print "d = $d\n";
}

sbr();
[download]

Thanks for correcting my mistake