'NaN' string goes through with '%.3E' format specifier on my old v5.8.8. 
my $missing = "NaN";
my %h = (
    A   => 1.23435
);
my $out = sprintf "%.3E", $h{A} || $missing;
warn $out;
$out = sprintf "%.3E", $h{B} || $missing;
warn $out;

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In reply to Re: Printing non existing hash values as "missing" by pme
in thread Printing non existing hash values as "missing" by Anonymous Monk

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