I believe that in those examples, Perl is actually leaking the scalars, along with a lot of other stuff -- look at the "still reachable" stat. It's doing this intentionally, by exiting immediately rather than painstakingly deallocating everything first.
In order to persuade Perl to deallocate everything, you need a Perl compiled with -DDEBUGGING and to set the PERL_DESTRUCT_LEVEL environment variable to 2. You are also likely need to create a Valgrind suppressions file to adapt for miscellaneous noise from libc, Dynaloader, etc.
In the end, though, you still don't see any difference when running those two examples. I would argue that the proof needed tightening up -- but the reasoning and explanation were sound.
marvin@smokey:~ $ PERL_DESTRUCT_LEVEL=2 valgrind --show-reachable=yes
+--leak-check=full \
> --suppressions=~/debugperl.supp \
> debugperl -e'my $x; $x = \$x; undef $x if $ARGV[0]' 1
==79872== Memcheck, a memory error detector
==79872== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et
+al.
==79872== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright
+ info
==79872== Command: debugperl -emy\ $x;\ $x\ =\ \\$x;\ undef\ $x\ if\ $
+ARGV[0] 1
==79872==
--79872-- /Users/marvin/bin/debugperl:
--79872-- dSYM directory has wrong UUID; consider using --dsymutil=yes
==79872==
==79872== HEAP SUMMARY:
==79872== in use at exit: 94,131 bytes in 12 blocks
==79872== total heap usage: 625 allocs, 613 frees, 219,612 bytes all
+ocated
==79872==
==79872== LEAK SUMMARY:
==79872== definitely lost: 0 bytes in 0 blocks
==79872== indirectly lost: 0 bytes in 0 blocks
==79872== possibly lost: 0 bytes in 0 blocks
==79872== still reachable: 0 bytes in 0 blocks
==79872== suppressed: 94,131 bytes in 12 blocks
==79872==
==79872== For counts of detected and suppressed errors, rerun with: -v
==79872== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from
+0)
marvin@smokey:~ $ PERL_DESTRUCT_LEVEL=2 valgrind --show-reachable=yes
+--leak-check=full \
> --suppressions=~/debugperl.supp \
> debugperl -e'my $x; $x = \$x; undef $x if $ARGV[0]' 0
==79881== Memcheck, a memory error detector
==79881== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et
+al.
==79881== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright
+ info
==79881== Command: debugperl -emy\ $x;\ $x\ =\ \\$x;\ undef\ $x\ if\ $
+ARGV[0] 0
==79881==
--79881-- /Users/marvin/bin/debugperl:
--79881-- dSYM directory has wrong UUID; consider using --dsymutil=yes
==79881==
==79881== HEAP SUMMARY:
==79881== in use at exit: 94,131 bytes in 12 blocks
==79881== total heap usage: 625 allocs, 613 frees, 219,612 bytes all
+ocated
==79881==
==79881== LEAK SUMMARY:
==79881== definitely lost: 0 bytes in 0 blocks
==79881== indirectly lost: 0 bytes in 0 blocks
==79881== possibly lost: 0 bytes in 0 blocks
==79881== still reachable: 0 bytes in 0 blocks
==79881== suppressed: 94,131 bytes in 12 blocks
==79881==
==79881== For counts of detected and suppressed errors, rerun with: -v
==79881== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from
+0)
marvin@smokey:~ $
