This is because $thing could be defined but zero and we don't want to overwrite a perfectly valid zero from the command line.
As
Not_a_Number points out, the syntax
EXPR unless defined $thing will do nothing at all * if
$thing is defined, whether it's true or false. That is, if we are executing
EXPR, then
$thing is guaranteed undefined, hence false; so
$thing ||= 'default' is guaranteed to be the same as
$thing = 'default'.
It seems reasonable to guess that what happened is that the coder originally had $thing ||= 'default' in some old code, discovered (as you mention) that it doesn't work when $thing is false-but-defined, and added the defined check without realising that it made ||= redundant.
Of course, a mere five years later, we have the wonderful //= (C style Logical Defined Or) instead to save us this pain.
UPDATE (the *'d statement above—sorry, I don't know how to do footnotes): On further thought, it's not quite true that EXPR unless defined $thing will do nothing if $thing is defined. Among what I suppose are many other subtle cases, if the unless is the last line in a subroutine, it'll make the subroutine return 1 when $thing is defined. For example, after
our $a = 1;
sub b { 0 unless defined $a }
sub c {}
my $b = b;
my $c = c;
we have that
$b = 1 but
$c is undefined.
