Implemented suggestion by Neil Hooey to check for and report

undefined message elements before they're assembled in Appender.pm
and a warning is issued without a proper code location:
mschilli#15

Changes

@@ -6,6 +6,10 @@
    *    (ms) Meir Guttman reported a use case for logging messages
              with dynamic levels on log4perl-devel@lists.sourceforge.net.
              Added to manual page.
+   *    (ms) Implemented suggestion by Neil Hooey to check for and report
+             undefined message elements before they're assembled in Appender.pm
+             and a warning is issued without a proper code location:
+             https://github.com/mschilli/log4perl/issues/15
 
 1.37 (2012/05/30)
    *    (ms) [rt.cpan.org #75655] Meir Guttman found the module to make

lib/Log/Log4perl/Appender.pm

@@ -6,8 +6,9 @@ use 5.006;
 use strict;
 use warnings;
 
-use Log::Log4perl::Level;
 use Log::Log4perl::Config;
+use Log::Log4perl::Level;
+use Carp;
 
 use constant _INTERNAL_DEBUG => 0;
 
@@ -162,10 +163,20 @@ sub log {
             #not defined, the normal case
         if (! defined $self->{warp_message} ){
                 #join any message elements
-            $p->{message} = 
-                join($Log::Log4perl::JOIN_MSG_ARRAY_CHAR, 
-                     @{$p->{message}} 
-                     ) if ref $p->{message} eq "ARRAY";
+            if (ref $p->{message} eq "ARRAY") {
+                for my $i (0..$#{$p->{message}}) {
+                    if( !defined $p->{message}->[ $i ] ) {
+                        local $Carp::CarpLevel =
+                        $Carp::CarpLevel + $Log::Log4perl::caller_depth + 1;
+                        carp "Warning: Log message argument #" . 
+                             ($i+1) . " undefined";
+                    }
+                }
+                $p->{message} = 
+                    join($Log::Log4perl::JOIN_MSG_ARRAY_CHAR, 
+                         @{$p->{message}} 
+                         );
+            }
 
             #defined but false, e.g. Appender::DBI
         } elsif (! $self->{warp_message}) {

t/065Undef.t

@@ -0,0 +1,28 @@
+use strict;
+
+use File::Temp qw( tempfile );
+
+BEGIN { 
+    if($ENV{INTERNAL_DEBUG}) {
+        require Log::Log4perl::InternalDebug;
+        Log::Log4perl::InternalDebug->enable();
+    }
+}
+
+my($tmpfh, $tempfile) = tempfile( UNLINK => 1 );
+
+use Test::More;
+BEGIN { plan tests => 1 };
+use Log::Log4perl qw( :easy );
+
+Log::Log4perl->easy_init( { level => $DEBUG, file => $tempfile } );
+
+my $warnings = "";
+
+$SIG{__WARN__} = sub {
+   $warnings .= $_[0];
+};
+
+DEBUG "foo", undef, "bar";
+
+like $warnings, qr/Log message argument #2/, "warning for undef element issued";