use warnings;
use strict;

my $value       = 0;
my $from        = '';
my $to          = '';
my $prefix      = '';
my %prefixes    = ();

%prefixes    = (
   mega      => 6,
   kilo      => 3,
   milli     => -3,
   micro     => -6,
   nano      => -9,
   pico      => -12,
   femto     => -15,
   atto      => -18,
   zepto     => -21,
   yocto     => -24,
);

print "Enter the unit that you are starting with: ";
chomp( $from = <STDIN> );

print "Enter your target unit: ";
chomp( $to = <STDIN> );

print "Enter the numerical amount: ";
chomp( $value = <STDIN> );

if ( not exists $prefixes{$from} ) {
    die qq/I don't know about the prefix $from\n/;
}

if ( not exists $prefixes{$to} ) {
    die qq/I don't know about the prefix $to\n/;
}

$prefix = $prefixes{$from} - $prefixes{$to};

print "$value $from is ", $value * (10**$prefix), " $to. \n";

#Output:
C:\perl pm_test.pl

Enter the unit that you are starting with: mega
Enter your target unit: kilo
Enter the numerical amount: 10
10 mega is 10000 kilo.
[download]