The main problem is that you are never actually setting the values for $to and $from. Below, I've made a couple changes to your script so that these values are set. Also, I made a couple other changes, including adding a note so that the user gets a prompt when entering an unrecognized currency and chomping $value so that the formatting of the output looks as you wanted it to. 
#!/usr/bin/env perl
# convert3.pl
use warnings;
use strict;

my ( $value, $from, $to, $rate, $rates, %rates );
%rates = (
    pounds          => 1,
    USD             => 1.6,
    marks           => 3.0,
    "french francs" => 10.0,
    yen             => 174.8,
    "swiss francs"  => 2.43,
    drachma         => 492.3,
    euro            => 1.5
);
print "Enter your starting currency: ";
OUTER0: while (<STDIN>) {
    chomp;
  INNER0: for my $cur (keys %rates) { # added keys so you don't loop t
+hrough values, too
        $from = $cur; # set $from currency
        last OUTER0 if $from eq $_;
    }
    print "Currency not recognized, try again: "; # Added to avoid con
+fusion when an unknown currency is entered
}
print "Enter your target currency: ";
OUTER1: while (<STDIN>) {
    chomp;
  INNER1: for my $cur (keys %rates) { # added keys so you don't loop t
+hrough values, too
        $to = $cur; # set $to currency
        last OUTER1 if $to eq $_;
    }
    print "Currency not recognized, try again: "; # Added to avoid con
+fusion when an unknown currency is entered
}
print "Enter your amount to convert: ";
$value = <STDIN>;
chomp $value; # added to fix format of output
$rate  = $rates{$to} / $rates{$from};
print "$value $from is ", $value * $rate, " $to. \n";

__END__
Enter your starting currency: french francs
Enter your target currency: swiss francs
Enter your amount to convert: 100
100 french francs is 24.3 swiss francs.

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In reply to Re: a hash and 2 loops--not working by frozenwithjoy
in thread a hash and 2 loops--not working by jhumphreys

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