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

##</code><code>##

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

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