In your mind, undef is some magical value. In mine, it's a function like any other.

Then my mind is right, and yours is wrong!

#! perl -slw
use 5.010;
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'uCmp', CLEAN_AFTER_BUILD => 0;

bool uCmp( SV* t ) {
    return (bool)( t == (&PL_sv_undef) );
}

END_C

print 'explicit undef: ', uCmp( undef ) ? 1 : 0;

my $s;
print 'uninitalised lexical: ', uCmp( $s ) ? 1 : 0;

my @a;
print 'uninitalised array element: ', uCmp( $a[ 5 ] ) ? 1 : 0;

my %h;
print 'uninitalised hash value: ', uCmp( $h{fred} ) ? 1 : 0;

our $g;
print 'uninitalised global: ', uCmp( $g ) ? 1 : 0;


print 'Other unexplicit undef values: ', uCmp( `unknown_command` ) ? 1
+ : 0;

__END__
C:\test>uCmp
explicit undef: 1
uninitalised lexical: 0
uninitalised array element: 0
uninitalised hash value: 0
uninitalised global: 0
'unknown_command' is not recognized as an internal or external command
+,
operable program or batch file.
Usage: main::uCmp(t) at C:\test\uCmp.pl line 28.
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