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]

I think you want a single minus sign for the exponent arithmetic. The final print needs some work as what it displays is misleading.

(You also need something like #!/usr/local/bin/perl as your first line.)

Cool. I have downloaded the TextMate text editor and now it runs, must have been an AdobeGoLive problem. I have searched for the OS units program, but don't seem to have it installed. Could you kindly provide a link? I have searched online but have not found a place to download it.

Eeww. Consider getting a real code editor (or learn a *NIX standard such as vim or emacs). XCode's builtin editor also would be a step up (presuming you have the OS X development tools installed).

And as a comment to your original program, you might consider using the OS' units program which already has a fairly good knowledge of conversions rather than reinventing the wheel piecemeal yourself.

$ units kilograms grams
        * 1000
        / 0.001
$ units feet kilometers
        * 0.0003048
        / 3280.8399
[download]

(Those two lines are what you multiply the later by to get the former and what you divide the former by to get the later)

Update: Duuur. Strike that, reverse it; as is noted below. Been so long since I've used it I forgot how it works. (At least I remembered it was there . . . :)

The cake is a lie.
The cake is a lie.
The cake is a lie.

I can run your script once I've added #!/usr/local/bin/perl as the first line (because I'm using Solaris).

What exactly is the error you're getting?

perl -c "labmonkey.pl"
Scalar found where operator expected at labmonkey.pl line 34, near "--
$allprefixes"
        (Missing operator before  $allprefixes?)
syntax error at labmonkey.pl line 34, near "-- $allprefixes"
labmonkey.pl had compilation errors.
[download]

I noticed that you didn't start the code with a 'shebang' line. May I suggest adding these lines to the top of your code:

#! /usr/local/bin/perl
use strict;
use warnings;
[download]

Welcome to the Monastery, have fun with the Toys, don't forget to enjoy yourself.

That error doesn't sound like something Perl would give. Is that the error message exactly? Are you sure perl is being used?

And line 1 isn't near EOF. Are the line endings correct? Again, Perl shouldn't care about that.

#!/usr/bin/perl
# (of course, "/usr/local/bin/perl" should work as well)

use strict;

if ( @ARGV != 3 ) {
    die "Usage: $0  starting_unit  target_unit  numeric_amount\n";
}
my ( $from, $to, $value ) = @ARGV;

#use these as exponents; below we just subtract exponents to get the a
+nswer
my %prefixes = (
    "mega"    => 6,  
    "kilo"    => 3,
    "milli"    => -3,
    "micro"    => -6,
    "nano"    => -9,
    "pico"    => -12,
    "femto"    => -15,
    "atto"    => -18,
    "zepto"    => -21,
    "yocto"    => -24,
);

for ( $from, $to ) {
    next if ( exists( $prefixes{$_} ));
    my $msg = "$_ is not a known unit.  Try one of these:\n";
    for my $prf (sort { $prefixes{$b}<=>$prefixes{$a} } keys %prefixes
+ ) {
        $msg .= "\t$prf\n";
    }
    die $msg;
}

my $exp = $prefixes{$from} - $prefixes{$to};
print "$value $from is ", $value * 10 ** $exp, " $to.\n";
[download]

The virtues and bonuses of using @ARGV to get user input from args on the command line are numerous, and you will appreciate them more and more as time goes by.

Simple example: does your shell support "command history", where you are able to use the "up-arrow" key to get back to a previous command, edit it and run again? (All decent shells have this.) Consider wanting to run your script 5 times with different user inputs... what is easier: answering the same three questions repeatedly 5 times? or recalling a previous command, changing one or more args on the command line, and running it again? Try it both ways, and see which one you like better.