ok sure.. i've worked out something.. but having problems with it.
i decide to use substring to extract "ATG".

print "\nThe sequence is :$sequence\n\n";
@sequence = $sequence;
@sequence= split('', $sequence);

$start ="ATG";
$a1 = substr ($sequence, $start, 3);
print "$a1\n";
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it can't display all of the "ATG"s in the fasta file.. help please :(