Some of the checks are redundant. This

        if( ( $b >= 5 ) && ( $b < 7 ) ) {
            $c[$i] += sum @a[ $i-$b+1 .. $i ];
            $c[$i] /= $b;
        }
        elsif( ( $b >= 7 ) && ( $b < 9 ) ) {
            $d[$i] += sum @a[ $i-$b+1 .. $i ];
            $d[$i] /= $b;
        }
        elsif( ( $b >= 9 ) && ( $b < 15 ) ) {
            $e[$i] += sum @a[ $i-$b+1 .. $i ];
            $e[$i] /= $b;
        }
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can be reduced to

        if( $b < 5 ) {
        }
        elsif( $b < 7 ) {
            ( $c[$i] += sum @a[ $i-$b+1 .. $i ] ) /= $b;
        }
        elsif( $b < 9 ) {
            ( $d[$i] += sum @a[ $i-$b+1 .. $i ] ) /= $b;
        }
        elsif( $b < 15 ) {
            ( $e[$i] += sum @a[ $i-$b+1 .. $i ] ) /= $b;
        }
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