%nuc = (
'F' => [0.001, 0.001, ...],
...
)

##</code><code>##

my %mapping = (
    1 => "A",
    2 => "R",
    3 => "N",
    ...
);
if(exists $mapping{$names[$j]}) {
    $names[$j] = $mapping{$names[$j]};
}
else {
    # Default value (last else-Statement)
    $names[$j]="V";
}