$ perl -w -MData::Dumper -E'
 $SIG{__WARN__} = sub { print Dumper \@_, [ caller(0) ] }; # create and install the handler
 my $undef; # obviousely undefined
 say "$undef undef"; # warning occurs here
'
$VAR1 = [
          'Use of uninitialized value $undef in concatenation (.) or string at -e line 4.
'
        ];
$VAR2 = [
          'main',
          '-e',
          1,
          'main::__ANON__',
          1,
          '',
          undef,
          undef,
          133376,
          'UUUUUUUUUUUUU',
          {
            'feature_unicode' => 1,
            'feature_say' => 1,
            'feature_state' => 1,
            'feature_switch' => 1
          }
        ];
 undef