Ok, I wrote some simple code to show this, and managed to narrow the problem down a little. Here is the code:

This is the server:

#!/usr/bin/perl -w

use strict;
use Frontier::Daemon;
use Digest::MD5 qw/md5_hex/;

Frontier::Daemon->new (LocalPort => 12345,
                       methods =>{md5=>sub {return md5_hex (shift)}});

__END__
[download]

And the client:

#!/usr/bin/perl -w

use strict;
use Frontier::Client;

my $rpc = Frontier::Client->new (url =>"http://localhost:12345/RPC2");
print $rpc->call ("md5", "1234"), $/;

__END__
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For reference:

md5 of "1234": 81dc9bdb52d04dc20036dbd8313ed055
md5 of "": d41d8cd98f00b204e9800998ecf8427e

Now, here's what I found. If I run the server on my desktop (perl 5.8.0), I get the expected result (81dc..). If I run the server on an older system (perl 5.6.1), I get the erroneous one (d41d..), along with the error on the console:

Use of uninitialized value in subroutine entry at ./srv line 8.

I have also found that even in the older system, the following code works:

sub my_md5_hex {
    my ($str) = @_;
    my $cmd = "perl -MDigest::MD5 -e " .
        "'print Digest::MD5::md5_hex (\"$str\"), \$/'";
    my $md5 = `$cmd`;
    chomp ($md5);
    return $md5;
}
[download]

It's pretty gross, though.