My guess would be that it's complaining about undefined CFLAGS / CPPFLAGS / LDFLAGS environment variables (see line 19 and 20). Line numbers reported are not always precise when a statement stretches over multiple lines.

Try setting those env variables to nothing, i.e. export CFLAGS= etc., and see what happens...

  9 use Inline (
  10         C => "DATA",
  11         ($ENV{LIBUSB_LIBDIR}
  12             ? ( LIBS => "-L\"$ENV{LIBUSB_LIBDIR}\" " .
  13                         ($^O eq 'MSWin32' ? ' -llibusb -L\"$ENV{W
+INDDK}\\lib\\crt\\i386\" -lmsvcrt ' : '-lusb') )
  14             : ( LIBS => '-lusb', )
  15         ),
  16         ($ENV{LIBUSB_INCDIR} ? ( INC => "-I\"$ENV{LIBUSB_INCDIR}\
+"" ) : () ),
  17         NAME => 'Device::USB',
  18         VERSION => '0.35',
  19         CCFLAGS => "$Config{ccflags} $ENV{CFLAGS} $ENV{CPPFLAGS}"
+,
  20         LDDLFLAGS => "$Config{lddlflags} $ENV{LDFLAGS}",
  21    );
  22 print "CCFLAGS =>|$Config{ccflags}|$ENV{CFLAGS}|$ENV{CPPFLAGS}|\n
+";
  23 print "LDDLFLAGS =>|$Config{lddlflags}|$ENV{LDFLAGS}|\n";
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this gave the following errors and output:

root@gk-kubuntu-dev:~/dav/Nevada# perl kk.pl
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 10.
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 10.
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 10.
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 22.
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 22.
CCFLAGS =>|-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fstack-protector -fno-
+strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FIL
+E_OFFSET_BITS=64|||
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 23.
LDDLFLAGS =>|-shared -L/usr/local/lib -fstack-protector||
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This indicates $ENV{CFLAGS},$ENV{CPPFLAGS} and $ENV{LDFLAGS} are uninitialized. I'll need to determine what they are supposed to be. There is not much useful I can find. I wonder why the package did not set the variables it needed. Could it be the botched cpan install? Frankly I have no idea what those things should be. I will try defining them as '' or some value

 9 $ENV{CFLAGS} = 'A';
  10 $ENV{CPPFLAGS} = 'B';
  11 $ENV{LDFLAGS} = 'C';
  12 $ENV{LIBUSB_LIBDIR} = 'D';
  13 $ENV{WINDDK} = 'E';
  14 $ENV{LIBUSB__INCDIR} = 'F';
  15 use Inline ( 
  16         C => "DATA",
  17         ($ENV{LIBUSB_LIBDIR}
  18             ? ( LIBS => "-L\"$ENV{LIBUSB_LIBDIR}\" " .
  19                         ($^O eq 'MSWin32' ? ' -llibusb -L\"$ENV{W
+INDDK}\\lib\\crt\\i386\" -lmsvcrt ' : '-lusb') )
  20             : ( LIBS => '-lusb', )
  21         ),
  22         ($ENV{LIBUSB_INCDIR} ? ( INC => "-I\"$ENV{LIBUSB_INCDIR}\
+"" ) : () ),
  23         NAME => 'Device::USB',
  24         VERSION => '0.35',
  25         CCFLAGS => "$Config{ccflags} $ENV{CFLAGS} $ENV{CPPFLAGS}"
+,
  26         LDDLFLAGS => "$Config{lddlflags} $ENV{LDFLAGS}",
  27    ); 
  28 print "CCFLAGS =>|$Config{ccflags}|$ENV{CFLAGS}|$ENV{CPPFLAGS}|\n
+";
  29 print "LDDLFLAGS =>|$Config{lddlflags}|$ENV{LDFLAGS}|\n";
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gives the following errors and output

Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 16.
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 16.
Use of uninitialized value in concatenation (.) or string at /usr/lib/
+perl5/Device/USB.pm line 16.
CCFLAGS =>|-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fstack-protector -fno-
+strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FIL
+E_OFFSET_BITS=64|A|B|
LDDLFLAGS =>|-shared -L/usr/local/lib -fstack-protector|C|
[download]

from what I can tell, every ENV variable is assigned something, Why are there still 3 concatenation errors?

I'll need to determine what they are supposed to be.

I don't think they have to be set to anything. The idea presumably is that in case they are set they should be appended to whatever CCFLAGS / LDDLFLAGS options have been determined via %Config...

IMHO, the code should either test if those vars are defined before interpolating them, or disable warnings :)

As for your update:

from what I can tell, every ENV variable is assigned something, Why are there still 3 concatenation errors?

If you set them within the Perl source, you'll need to set them in a BEGIN { } block, because due to the use Inine (...) statement, they're being evaluated at compile time of the Perl code.

...
BEGIN {
  $ENV{CFLAGS} = '';
  $ENV{CPPFLAGS} = '';
  $ENV{LDFLAGS} = '';
}
use Inline (
        ...
        ... 
    );
...
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