I don't know where you get a 50% hit for the 'my' inside the for loop. When I do this:

$a_huge_value = 3200000;

my($foo);

$time1 = time();
for ($i = 0; $i < $a_huge_value; $i++) {
  $foo = getValue();
  codeThatUsesFoo($foo);
}

$time2 = time();
for ($i = 0; $i < $a_huge_value; $i++) {
   my $foo = getValue();
   codeThatUsesFoo($foo);
}

$time3 = time();
print $time2 - $time1, "\n";
print $time3 - $time2, "\n";

sub getValue { return $_[0]; }
sub codeThatUsesFoo { return $_[0]; }
[download]

I get:

[bobn@trc2:/home/bobn/misc]# perl my2.pl
14
14
[bobn@trc2:/home/bobn/misc]# perl my2.pl
13
15
[bobn@trc2:/home/bobn/misc]# perl my2.pl
13
15
[bobn@trc2:/home/bobn/misc]# perl my2.pl
14
16
[download]

Changing the subs to:

sub getValue { return }
sub codeThatUsesFoo { return }
[download]

yields:

[bobn@trc2:/home/bobn/misc]# perl my2.pl
12
12
[bobn@trc2:/home/bobn/misc]# perl my2.pl
12
13
[bobn@trc2:/home/bobn/misc]# perl my2.pl
13
13
[bobn@trc2:/home/bobn/misc]# perl my2.pl
12
13
[download]

SO the decision is really based on what it *should* be based on - is $foo intended to be used outside the loop, with the effects of the loop desired to be visible. If so, do it the first way. If not, do it the second way.