Premature optimization is the root of all evil.

Re 1: 

use strict;
use Benchmark;

my $a_huge_value = 10000000;

sub outside {
    my($foo);
    
    for (my $i = 0; $i < $a_huge_value; $i++) {
        $foo = $i + 1;
        $foo++;
    }
}

sub inside {
    for (my $i = 0; $i < $a_huge_value; $i++) {
        my $foo = $i + 1;
        $foo++;
    }
}

timethese 1, {
 inside => \&inside,
 outside => \&outside,
};
__END__

Benchmark: timing 1 iterations of inside, outside...
    inside:  6 wallclock secs ( 5.99 usr +  0.00 sys =  5.99 CPU) @  0
+.17/s (n=1)
   outside:  5 wallclock secs ( 4.95 usr +  0.01 sys =  4.96 CPU) @  0
+.20/s (n=1)

[download]
I don't think the difference is big enough. Especialy since the loops were real tight in the benchmark. If you actually call a subroutine in the loop you get much closer results. If I relace the $foo++ by a call to sub doSomething { $_[0]++ } I get: 
Benchmark: timing 1 iterations of inside, outside...
    inside: 12 wallclock secs (10.95 usr +  0.00 sys = 10.95 CPU) @  0
+.09/s (n=1)
   outside: 10 wallclock secs ( 9.49 usr +  0.02 sys =  9.51 CPU) @  0
+.11/s (n=1)

[download]
Anyway if you really do care about this tiny difference I would suggest a slightly different syntax: 
sub inInit {
    for (my ($i, $foo) = (0, 0); $i < $a_huge_value; $i++) {
        $foo = $i + 1;
        $foo++;
    }
}

[download]
This way the $foo is created just once, yet it's declared just for the loop. You should be aware though that under some circumstances it may make a huge difference whether the variable is declared outside or inside the loop. If you plan to keep a reference to the variable you do need to declare it inside the loop so that you do get a new variable in each iteration!

Re 2: First, you do not have any localy scoped variables in your code. You have a few LEXICALY scoped ones. Anyway all you need to notice is that 

for ($i = 0; $i < $a_whole_lot; $i++) {
  ($key, $value) = returnsAnArray();
}

[download]
is valid Perl. You do not need the declaration to be allowed to assign to a list of variables! So there is actually no difference between this and the first question.

Jenda
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
   -- Rick Osborne

Edit by castaway: Closed small tag in signature

In reply to Re: Two Questions on "my" by Jenda
in thread Two Questions on "my" by C_T

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