Write a Perl script that counts and prints the number of A's in a DNA
sequence. The input is one sequence in FASTA format in a file called
"dna.txt". 

For example if the file contains 

>human
ACCGT

then the output of the program should be 1.

Submit your solution as a file called <firstname>_count.pl. Replace <firstname>
with your first name.

Usman