Does pre declaring variables before you use them have any memory/performance benefit for the interpreter by providing type information.

Essentially no. There are several reasons for this one of which is that perl isn't a strongly typed language so the compiler isn't designed to take advantage of 'type hints'. In fact it is every so slightly quicker to *not* predeclare your variables in terms of execution speed as it increases the amount of operations to be performed

use Benchmark qw(cmpthese);

## take the results with a grain of salt, mmmkay?
cmpthese(-15, {
    predeclare => sub { my $x = {}; $x->{foo} = "bar" x 20 },
    nodeclare  => sub { my $x; $x->{foo} = "bar" x 20 }
});

__output__

Benchmark: running nodeclare, predeclare, each for at least 15 CPU sec
+onds...
 nodeclare: 24 wallclock secs (15.33 usr +  0.00 sys = 15.33 CPU) @ 11
+0465.75/s (n=1693440)
predeclare: 18 wallclock secs (15.58 usr +  0.02 sys = 15.60 CPU) @ 89
+620.83/s (n=1398085)
               Rate predeclare  nodeclare
predeclare  89621/s         --       -19%
nodeclare  110466/s        23%         --
[download]

So as you can see the sub *without* the anonymous hash assignment was ever so slightly faster, but I'd say the difference is fairly negligible.

I know in other strongly typed languages you should always declare the type of your variable but does it matter in perl or am I doing it by prepending @ or % or $.

There are only really two types in perl and they are scalars and lists and everything else from there on out is an abstraction. For more information on strongly-typed languages and perl see. Ovid's Griping about Typing, and for info about data types in perl see the perldata manpage.

Also is putting a 'my' call inside a loop to clear the variable advisable?

I think you may have a misunderstanding of what function my() performs. Declaring a variable with my() will create a new variable in the current lexical scope, that's all. So you're not 'clearing' the variable you're just creating a new variable in the current lexical scope e.g

my $foo = "a string";
print "\$foo is $foo\n";

## this $foo only lasts to the end of the for loop scope
for my $foo (1..3) {
    print "\$foo is $foo\n";
}

print "\$foo is $foo\n";

__output__

$foo is a string
$foo is 1
$foo is 2
$foo is 3
$foo is a string
[download]

HTH

_________
broquaint