You may be looking for something like this:

...
print "Please enter RNAi name, concentration, volume:  (empty line to 
+continue)\n";

while ((my $line = <STDIN>) ne "\n") {
    chomp $line;
    my ($name, $conc, $vol) = split ' ', $line;  # split by whitespace

    $RNA{$name} = $conc;
    $vol{$name} = $vol;
}
...
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