Answer 1
I would use the second variation, because it hides $foo from the rest of the code, eliminating the possibility of side-effects from having $foo still visible. The code generated by this Perl reuses your scoped $foo anyway as an optimization, so there is no penalty.

Note that my ($foo); could be more clearly represented as my $foo; - the parens are usually used in declarations to collapse several lines into one, such as:

  my ($a, $b, $c);
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or

  my ($a, $b, $c) = (0, 1, 2);
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or to capture a subroutine's arguments

  my ($a, $b) = @_;
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Answer 2
The declaration of $key and $value both outside and inside the loop is redundant, but I see nothing wrong with your code inside the loop, other than the fact that the loop control variable $i is not referenced in the body, so the loop could be rewritten as:

for (1 .. $a_whole_lot)
{
  my ($key, $value) = returnsAnArray ();
}
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Again, the scoped variables are going to be reused.

Aside
In question 1, your code doesn't even need a $foo.

  codethatUsesFoo (getValue()) for 1 .. $a_huge_value;
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In general, you are using the C-style for loop, and it would be more Perlish of you to use for or foreach as shown.

Hope this helps.