I hope the previous replies have brought you to a point where you have a functioning and satisfying script. Now, I'd like to introduce you to my best friend... @ARGV, meet theredqueentheory. Red, meet @ARGV: 
#!/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]
I think that produces the sort of result you were looking for (I adopted the mathematical corrections provided by DigitalKitty).

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.

In reply to Re: syntax error at labmonkey.pl line 1, at EOF by graff
in thread syntax error at labmonkey.pl line 1, at EOF by theredqueentheory

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