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my ( $prev, %pairs );
map { $pairs{ $prev . $_}++; $prev = $_; } @genome;
delete $pairs{a}, $pairs{c}, $pairs{t}, $pairs{g};

my ( $prev, %pairs );
for ( @genome ) {
...
    $prev = $_; 
};
delete $pairs{a}, $pairs{c}, $pairs{t}, $pairs{g};