Hi again, I'm not sure whether What could we do with this? refers to the sub after the question, or the way Perl disallows certain characters in variable names. If the latter, I would suggest that independent of your variable naming issue, you consider extensibility of your program. There doesn't seem to be much advantage to a sub named $_to_¥ as compared to a more generic one that takes arguments:

use strict;
use warnings;
use Test::More;
use Test::More::UTF8; # loads utf8.pm

sub convert_currency {
    my %map = (
        '$¥' => sub { $_[0] * 105.4 },
        '¥$' => sub { $_[0] * 0.0095 },
        '$€' => sub { $_[0] * 0.89 },
        '€$' => sub { $_[0] * 1.12 },
    );

    my ($from, $to, $amount) = @_;

    my $converter = $map{"$from$to"};

    return sprintf( '%.3f', $converter->($amount) );
}

is( convert_currency('$', '¥', 5),     '527.000', '$ -> ¥' );
is( convert_currency('€', '$', 42.02),  '47.062', '€ -> $' );
is( convert_currency('$', '€', 42.02),  '37.398', '$ -> €' );

done_testing;
[download]

$ prove -v monks/11104981.pl

monks/11104981.pl ..
ok 1 - $ -> ¥
ok 2 - € -> $
ok 3 - $ -> €
1..3
ok
All tests successful.
Files=1, Tests=3,  0 wallclock secs ( 0.02 usr  0.00 sys +  0.04 cusr 
+ 0.00 csys =  0.06 CPU)
Result: PASS
[download]

Hope this helps!

I wanted to say that currency symbol are not allowed in names because $ is a sigil too.