A friend of mine is living this year in the USA, and she asked me how to program with Perl some unit conversions. The currency conversion is volatile since the ratio changes a lot, so I came up with this:

use strict;
use warnings;

use Locale::Currency;
use File::Basename;
use Cache::FileCache;
use Finance::Quote;

die "usage: $0 amount currency_from currency_to\n"
    unless @ARGV == 3;

my $amount = shift @ARGV;

my ($currency_from, $currency_to) = map { uc } @ARGV;

for my $code ($currency_from, $currency_to) {
    die "sorry, currency $code not in ISO 4217\n"
        unless defined code2currency($code);
}

my ($filename) = fileparse($0, '.pl');

my $cache = Cache::FileCache->new({
    cache_root         => "$ENV{HOME}/.$filename",
    default_expires_in => '1 day',
});

my $quote = Finance::Quote->new();

my $ratio = $cache->get("$currency_from:$currency_to");

$ratio = $quote->currency($currency_from, $currency_to)
    unless defined $ratio;

die "sorry, cannot convert from $currency_from to $currency_to\n"
    unless defined $ratio;

$cache->set("$currency_from:$currency_to", $ratio);
print "$amount $currency_from = ", $amount * $ratio, " $currency_to\n"
+;
[download]

She liked it, and so do I because it's so simple that it doesn't need comments at all. Just try:

$ perl exchange.pl 100 usd eur
100 USD = 70.22 EUR
[download]

Update: added ISO 4217 currency code check per graff's request.