How about:

my $out = exists $h{$_} ? sprintf "%.3E", $h{$_} : 'MISSING';
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As in:

my %h = (
    A   => 1.23435,
    C   => 6.78901,
);

for ('A' .. 'C') {
    my $out = exists $h{$_} ? sprintf "%.3E", $h{$_} : 'MISSING';
    warn $out;
}
[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)