Well,
1) is easy. Use the modulo operator %
It returns the remainder after a division so $number % 2 is only zero for even numbers and is 1 for odd numbers. See the perlop page for lots of info.

2) is a bit harder. I'm sure there are many elegant solutions. Some probably got posted while I was looking into it. My first try, if the files aren't too big, would be to slurp them into arrays and then fold them together.

use strict;
use warnings;

open (T1, "t1") or die;
my @file1 = <T1>;
my @file2 = <DATA>;
my @file;

while (@file1 and @file2) {
  push @file, (shift @file1, shift @file2);
}

push @file, @file1 or @file2; #catch any leftover

while (my $line = shift @file) {
   #do 'things' with it
    print $line;
}

__END__
file1
file1
file1
file1
[download]

the t1 file looks like the END block but with 2 instead of 1 I get

Useless use of private array in void context at junk line 13.
file2
file1
file2
file1
file2
file1
file2
file1
file2
file3, kidding... file2
[download]

Not sure what the warnings for, I guess because one of the array's is empty maybe? I'll bet if you put that into an if/else it wouldn't complain.

There's probably a more elegant solution, but this is what comes to mind.

3) Did you try it? I just ran the code you posted (cleaned up with ';' :-) and got this

use strict;
use warnings;

$/ = "****";

my @array;

while (<DATA>) {
     chomp;
     push @array, $_;
     $/= "####";
}

print "@array";

__END__
stuff****more
stuff####less
[download]

and got

$ perl junk
stuff more
stuff less
[download]

Hope this helps
Ira

"So... What do all these little arrows mean?"
~unknown