Each value is a single value. However that single value is allowed to be a reference.

Try something like:

while (not $done)
{
  my $name = 'Placebex';
  my %info;
  $info{concentration} = 0;
  $info{volume} = 250;

  $recipe{name} = \%info;
}

foreach my $name (keys %recipe)
{
  print "Take $recipe{$name}{volume} ml of $name,\n";
}
print "and call me in the morning.\n"
[download]

You might want to use a hash of hashes (HoH) data structure to hold all of your data in one place. You can also use a while loop to read from STDIN until some end condition - here I use CTRL-D to signal eof as I'm on *nix. The Data::Dumper module is helpful when trying to visualise data and I think printf (and sprintf) will do your rounding for you.

#!/usr/bin/perl
#
use strict;
use warnings;

use feature qw{ say };

use Data::Dumper;

my $prompt = q{-> };
my $dePrompt
   = qq{\r}
   . q{ } x ( length( $prompt ) + 2 )
   . qq{\r};
my %RNA;

print qq{Please enter name, conc. and vol. (CTRL-D to finish):\n};
print $prompt;

while ( 1 )
{
    do { print $dePrompt; last; } if eof STDIN;
    chomp( my $line = <STDIN> );
    my( $name, $conc, $vol ) = split m{\s+}, $line;
    $RNA{ $name } = {
       conc => $conc,
       vol  => $vol,
       };
    print $prompt;
}

print Data::Dumper->Dumpxs( [ \ %RNA ], [ qw{ *RNA } ] );
say q{-} x 50;

foreach my $name ( sort keys %RNA )
{
    printf qq{%10s%8.2f%6d%8.2f%8.2f%8.2f\n},
       $name,
       $RNA{ $name }->{ conc },
       $RNA{ $name }->{ vol },
       $RNA{ $name }->{ conc } * $RNA{ $name }->{ vol },
       30 / $RNA{ $name }->{ conc },
       50 / $RNA{ $name }->{ conc };
}
[download]

Here's a run of it.

Please enter name, conc. and vol. (CTRL-D to finish):
-> Ash2 1.85 150
-> Ja2_1 1.89 150
-> Ja2_2 2.49 100
-> Set1_1 1.55 150
-> Set1_2 2.88 100
%RNA = (
         'Set1_2' => {
                       'vol' => '100',
                       'conc' => '2.88'
                     },
         'Ja2_1' => {
                      'vol' => '150',
                      'conc' => '1.89'
                    },
         'Ash2' => {
                     'vol' => '150',
                     'conc' => '1.85'
                   },
         'Set1_1' => {
                       'vol' => '150',
                       'conc' => '1.55'
                     },
         'Ja2_2' => {
                      'vol' => '100',
                      'conc' => '2.49'
                    }
       );
--------------------------------------------------
      Ash2    1.85   150  277.50   16.22   27.03
     Ja2_1    1.89   150  283.50   15.87   26.46
     Ja2_2    2.49   100  249.00   12.05   20.08
    Set1_1    1.55   150  232.50   19.35   32.26
    Set1_2    2.88   100  288.00   10.42   17.36
[download]

Note that my use of ^D and the $dePrompt string tidies away the last prompt. I hope this is helpful.

The Data::Dumper module is helpful when trying to visualise data

Or see How can I visualize my complex data structure?

I believe the OP would also benefit from perusing our very fine Tutorials, esp. Getting Started with Perl.

HTH,

planetscape

chomp( my $line = <STDIN> );
print "You typed '$line'.\n";
[download]

three inputs is like: jar, 2, 200 or separate by tab

how to store three inputs on same line into three scalar variable?

and loop on several lines of inputs?

how to store three inputs on same line into three scalar variable?

split

and loop on several lines of inputs?

open a file, or...

while (<>) {
   # do something
}
[download]

See also perlintro

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;
}
...
[download]