@temp = (); only empties the array, undef @temp; completely removes it from memory. For the latter, the final memory consumption will be ever so slightly less. If you want to use the array again, the former will be just a teeny weeny little bit faster.

But like I said, don't break your head over it. The difference is negligible.

@tmp = () makes @tmp an array with zero elements. undef @tmp undefines @tmp. Check the output from:

my @foo = qw(foo bar baz);
@foo = ();
print "\@foo is defined.\n" if defined @foo;

my @bar = qw(foo bar baz);
undef @bar;
print "\@bar is defined.\n" if defined @bar;
[download]

That may work fine on Perl4 but:

perldoc -f defined
[snip]
      Use of "defined" on aggregates (hashes and arrays) is deprecated
+.
[download]

So don't do that. :-)

I wouldn't use something like that in production code, but it's good to know that there's a reason beyond my personality flaws. ;)

Owl looked at him, and wondered whether to push him off the tree; but, feeling that he could always do it afterwards, he tried once more to find out what they were talking about.