While his "at runtime" is a straw man, it's not as simple as you imply it is. A lot more than undef returns PL_sv_undef. For example, `foo` (that you insisted is completely unrelated to undef) also returns PL_sv_undef. 
#! 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;
print '`foo`:          ', uCmp( scalar(`foo`) ) ? 1 : 0;

__END__
explicit undef: 1
Can't exec "foo": No such file or directory at a.pl line 14.
`foo`:          1

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In reply to Re^3: Use of uninitialized value in substr by ikegami
in thread Use of uninitialized value in substr by BrowserUk

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