You can use the modulus operator for that.

my $is_even = $number % 2 == 0;
my $is_odd  = $number % 2 == 1;
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

Check the least significant bit. It is set for odd numbers:

print "$_ is " . ($_ & 1 ? "odd" : "even") . "\n" for 0..3;
[download]

Prints:

0 is even
1 is odd
2 is even
3 is odd
[download]

So, for example:

if (0 == $N % 2) {
    print "The number $N is even\n";    # There is no remainder
} else {
    print "The number $N is odd\n";     # There is a remainder (of 1)
}
[download]

If you want to count the number of occurences of a single character in a string, then you should look at using 'tr'.

$str = 'ABABAABBA';
$count = $str =~ tr/A//;  # $a = 5;
[download]

If you omit the second arg to tr, it knows you are not changing the string, but still counts up the occurences for you, which makes this very efficient.

After that you just have to see if the number is positive ($count > 0) and use Grandfather's test above to see if it is even ( !($count & 1) ).

if ($count && !($count & 1)) {
  # Do something with this protein sequence
}
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print $sequence if $sequence =~ /^[^A]*(?:A[^A]*A)+[^A]*$/;
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Dominus wrote a treatise that gives you the two functions odd and even which tell you whether a number is even or odd in his list of Infrequently Asked Questions about Perl.

I assume you are aware, (and it probably should be mentioned), that the referenced method is (I assume), a joke along the same lines as the included method of finding tomorrow's date:

sub tomorrow_date {
    sleep 86_400; 
    return localtime();
}
[download]

Without all the compile-time reductions, lazy evaluation etc. that most properly functional languages benefit from, this method of determining the eveness of a number is ludicrously inefficient--6000+ times slower than the normal method:

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];
no warnings 'recursion';

sub odd { my $number = shift; return !even ($number); }
sub even { my $number = abs shift; return 1 if $number == 0; return od
+d ($number - 1); }

cmpthese -5, {
    normal     => q[ my $n;     $_ & 1 and $n++ for 1 .. 1000; ],
    functional => q[ my $n; even( $_ ) and $n++ for 1 .. 1000; ],
};

__END__
c:\test>junk
              s/iter functional     normal
functional      1.50         --      -100%
normal     2.41e-004    625042%         --
[download]

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

Bioperl supports starting of EMBOSS programs and parsing output files.