Brethren, I seek the benefit of your experience.

I have a Perl application, a multithreaded client for the ICB chat service, to which I recently added client-to-client encryption as a feature.

Now, the base client requires only about two modules that are not normally part of the default Perl5 install. This, I considered acceptable. But adding encryption required the addition of about another eight modules. Since I did not know whether users would wish to install the additional eight modules for a feature they might not use, I sought a way to make use of those modules optional. And indeed, eventually I found such a means: Module::Load::Conditional.

After some study, I wrote the following code:

use Module::Load::Conditional qw[can_load check_install requires];

$Module::Load::Conditional::VERBOSE = 1;
my $crypt_mods = {'Crypt::CBC' => undef,
          'Crypt::Blowfish' => undef,
          'Digest::SHA1' => undef,
          'MIME::Base64' => undef,
          'Compress::Zlib' => undef,
          'Crypt::DH' => undef};

if ((  can_load(modules=> {'Math::BigInt::GMP' => undef}, verbose => 1
+)
    || can_load(modules=> {'Math::BigInt' => undef}, verbose => 1))
    && can_load(modules => $crypt_mods, verbose => 1))
{
    $encryption_avail = 1;
}

print $encryption_avail
      ? "All required crypto modules found and loaded: Encryption avai
+lable\n"
      : "Encryption disabled: required modules not found\n";
[download]

And lo, this code would reportedly load all the necessary modules if available, or, if one or more modules were not available, it would disable the encryption feature - but continue execution anyway.

Except that later on, a call to decode_base64() would fail.

Thread 3 terminated abnormally: Undefined subroutine &main::decode_base64 called at ./icbm.conditional-load line 2071.

That line is in the middle of the function below:

sub decrypt
{
    my ($message, $cipher, $key) = @_;
    my ($plaintext, $comptext, $ciphertext, $armortext, $cbc);

    $cbc = Crypt::CBC->new( -key    => $key,
                            -cipher => $cipher);
    $armortext = $$message;
    $ciphertext = decode_base64($armortext) || out ("Unarmor failed!")
+;
    $comptext = $cbc->decrypt($ciphertext) || out ("Decrypt failed!");
    $plaintext = Compress::Zlib::memGunzip($comptext) || out ("Decompr
+ess failed!");
    $$message = $plaintext;

    return (1);
}
[download]

But lo, did not Module::Load::Conditional say all modules had loaded? Doubting its word, I sought a means to verify that they were indeed loaded. I discovered that Module::Loaded, while it did not contain any function to directly tell me whether a module was loaded, would — if I instructed it to mark those modules as loaded — carp if it found them already loaded.

Thus I added the following code, right after the Module::Load::Conditional invocation above:

use Module::Loaded;
foreach my $mk ('Compress::Zlib', 'Crypt::Blowfish', 'Crypt::CBC', 'Cr
+ypt::DH',
                'Digest::SHA1', 'Math::BigInt::GMP', 'Math::BigInt', '
+MIME::Base64')
{
    mark_as_loaded($mk);
}
[download]

And lo, it reported all these modules were, indeed, already loaded, just as Module::Load::Conditional had said:

All required crypto modules found and loaded: Encryption available
'Compress::Zlib' already marked as loaded ('/usr/lib/perl5/5.10.0/i686
+-linux-thread-multi-64int-ld/Compress/Zlib.pm') at ./icbm.conditional
+-load line 225
'Crypt::Blowfish' already marked as loaded ('/usr/lib/perl5/site_perl/
+5.10.0/i686-linux-thread-multi-64int-ld/Crypt/Blowfish.pm') at ./icbm
+.conditional-load line 225
'Crypt::CBC' already marked as loaded ('/usr/lib/perl5/site_perl/5.10.
+0/Crypt/CBC.pm') at ./icbm.conditional-load line 225
'Crypt::DH' already marked as loaded ('/usr/lib/perl5/site_perl/5.10.0
+/Crypt/DH.pm') at ./icbm.conditional-load line 225
'Digest::SHA1' already marked as loaded ('/usr/lib/perl5/site_perl/5.1
+0.0/i686-linux-thread-multi-64int-ld/Digest/SHA1.pm') at ./icbm.condi
+tional-load line 225
'Math::BigInt::GMP' already marked as loaded ('/usr/lib/perl5/site_per
+l/5.10.0/i686-linux-thread-multi-64int-ld/Math/BigInt/GMP.pm') at ./i
+cbm.conditional-load line 225
'Math::BigInt' already marked as loaded ('/usr/lib/perl5/5.10.0/Math/B
+igInt.pm') at ./icbm.conditional-load line 225
'MIME::Base64' already marked as loaded ('/usr/lib/perl5/5.10.0/i686-l
+inux-thread-multi-64int-ld/MIME/Base64.pm') at ./icbm.conditional-loa
+d line 225
[download]

Brothers, I am perplexed. I am assured, doubly, that MIME::Base64 is loaded. And the other modules loaded with it are clearly loaded, or my code would fail much sooner (not least at the invocation of Crypt::CBC two lines earlier). Indeed, this is not even the first use of MIME::Base64 in the code.

So why, then, can I not call decode_base64(), when that code worked perfectly when I was loading it with a simple "Use MIME::Base64;"?