>perl -E '$x=undef; $y=$x // warn "RHS evaluated"; say $y'
RHS evaluated at -e line 1.
1
>perl -E '$x=""; $y=$x // warn "RHS evaluated"; say $y'

>perl -E '$x=0; $y=$x // warn "RHS evaluated"; say $y'
0
>perl -E '$x="true"; $y=$x // warn "RHS evaluated"; say $y'
true
>perl -E '$x=42; $y=$x // warn "RHS evaluated"; say $y'
42
>perl -V
Summary of my perl5 (revision 5 version 12 subversion 3) configuration:

  Platform:
    osname=linux, osvers=2.6.35.10, archname=x86_64-linux-thread-multi
    uname='linux midas64 2.6.35.10 #2 smp thu jan 6 19:06:19 cst 2011 x86_64 amd athlon(tm) ii x2 235e processor authenticamd gnulinux '
    config_args='-de -Dprefix=/usr -Dvendorprefix=/usr -Dcccdlflags=-fPIC -Dinstallprefix=/usr -Dlibpth=/usr/local/lib64 /usr/lib64 /lib64 -Doptimize=-O2 -fPIC -Dusethreads -Duseithreads -Dpager=/usr/bin/less -isr -Dinc_version_list=5.12.2 5.12.1 5.12.0 5.10.1 5.10.0 5.8.8 5.8.7 5.8.6 5.8.5 5.8.4 5.8.3 5.8.2 5.8.1 5.8.0 -Darchname=x86_64-linux'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2 -fPIC',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.5.2', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector'
    libpth=/usr/local/lib64 /usr/lib64 /lib64
    libs=-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lpthread -lc
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=/lib64/libc-2.12.2.so, so=so, useshrplib=false, libperl=libperl.a
    gnulibc_version='2.12.2'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -fPIC -fstack-protector'


Characteristics of this binary (from libperl):
  Compile-time options: MULTIPLICITY PERL_DONT_CREATE_GVSV
                        PERL_IMPLICIT_CONTEXT PERL_MALLOC_WRAP USE_64_BIT_ALL
                        USE_64_BIT_INT USE_ITHREADS USE_LARGE_FILES
                        USE_PERLIO USE_PERL_ATOF USE_REENTRANT_API
  Built under linux
  Compiled at Jan 26 2011 12:39:46
  %ENV:
    PERL_UNICODE="SDL"
  @INC:
    /usr/lib64/perl5/site_perl/5.12.3/x86_64-linux-thread-multi
    /usr/lib64/perl5/site_perl/5.12.3
    /usr/lib64/perl5/vendor_perl/5.12.3/x86_64-linux-thread-multi
    /usr/lib64/perl5/vendor_perl/5.12.3
    /usr/lib64/perl5/5.12.3/x86_64-linux-thread-multi
    /usr/lib64/perl5/5.12.3
    /usr/lib64/perl5/site_perl
    /usr/lib64/perl5/vendor_perl
    .