lc on variable with undefined content should remain undefined

Module:

Description When invoking the lc (lower-case) function on undefined content, I would expect undefined content. I'm getting an empty string instead.

Steps to Reproduce

package PerlTest 1.0;
use 5.010;
use strict;
use warnings;
use Carp;
use English '-no_match_vars';

sub print_defined {
  my ($v) = @_;
  my $d = ( defined $v ) ? 'DEFINED' : 'UNDEFINED';
  say $d or croak $OS_ERROR;
}

my $var;
print_defined($var);
my $result = lc $var;
print_defined($result);

1;

Expected behavior I'm expecting the script to produce UNDEFINED for both the initial variable $var, and the lower-cases $result.

Perl configuration


Summary of my perl5 (revision 5 version 26 subversion 1) configuration:

  Platform:
    osname=linux
    osvers=4.9.0
    archname=x86_64-linux-gnu-thread-multi
    uname='linux localhost 4.9.0 #1 smp debian 4.9.0 x86_64 gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fdebug-prefix-map=/build/perl-hx1dVS/perl-5.26.1=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.26 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.26 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.26 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.26.1 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.26.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.26.1'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='x86_64-linux-gnu-gcc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2 -g'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion=''
    gccversion='7.3.0'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='x86_64-linux-gnu-gcc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=libc-2.27.so
    so=so
    useshrplib=true
    libperl=libperl.so.5.26
    gnulibc_version='2.27'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -L/usr/local/lib -fstack-protector-strong'

Characteristics of this binary (from libperl): 
  Compile-time options:
    HAS_TIMES
    MULTIPLICITY
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_IMPLICIT_CONTEXT
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_ITHREADS
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
    USE_REENTRANT_API
  Locally applied patches:
    DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
    DEBPKG:debian/db_file_ver - https://bugs.debian.org/340047 Remove overly restrictive DB_File version check.
    DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information.
    DEBPKG:debian/enc2xs_inc - https://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @INC directories.
    DEBPKG:debian/errno_ver - https://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes.
    DEBPKG:debian/libperl_embed_doc - https://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking
    DEBPKG:fixes/respect_umask - Respect umask during installation
    DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories
    DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib
    DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor
    DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets.
    DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor.
    DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy.
    DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable.
    DEBPKG:debian/perlivp - https://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local
    DEBPKG:debian/deprecate-with-apt - https://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules
    DEBPKG:debian/squelch-locale-warnings - https://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts
    DEBPKG:debian/patchlevel - https://bugs.debian.org/567489 List packaged patches for 5.26.1-6ubuntu0.3 in patchlevel.h
    DEBPKG:fixes/document_makemaker_ccflags - https://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags}
    DEBPKG:debian/find_html2text - https://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text
    DEBPKG:debian/perl5db-x-terminal-emulator.patch - https://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl
    DEBPKG:debian/cpan-missing-site-dirs - https://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable
    DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] https://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected
    DEBPKG:debian/makemaker-pasthru - https://bugs.debian.org/758471 Pass LD settings through to subdirectories
    DEBPKG:debian/makemaker-manext - https://bugs.debian.org/247370 Make EU::MakeMaker honour MANnEXT settings in generated manpage headers
    DEBPKG:debian/kfreebsd-softupdates - https://bugs.debian.org/796798 Work around Debian Bug#796798
    DEBPKG:fixes/autodie-scope - https://bugs.debian.org/798096 Fix a scoping issue with "no autodie" and the "system" sub
    DEBPKG:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in Memoize
    DEBPKG:debian/hurd-softupdates - https://bugs.debian.org/822735 Fix t/op/stat.t failures on hurd
    DEBPKG:fixes/math_complex_doc_great_circle - https://bugs.debian.org/697567 [rt.cpan.org #114104] Math::Trig: clarify definition of great_circle_midpoint
    DEBPKG:fixes/math_complex_doc_see_also - https://bugs.debian.org/697568 [rt.cpan.org #114105] Math::Trig: add missing SEE ALSO
    DEBPKG:fixes/math_complex_doc_angle_units - https://bugs.debian.org/731505 [rt.cpan.org #114106] Math::Trig: document angle units
    DEBPKG:fixes/cpan_web_link - https://bugs.debian.org/367291 CPAN: Add link to main CPAN web site
    DEBPKG:fixes/time_piece_doc - https://bugs.debian.org/817925 Time::Piece: Improve documentation for add_months and add_years
    DEBPKG:fixes/extutils_makemaker_reproducible - https://bugs.debian.org/835815 https://bugs.debian.org/834190 Make perllocal.pod files reproducible
    DEBPKG:fixes/file_path_hurd_errno - File-Path: Fix test failure in Hurd due to hard-coded ENOENT
    DEBPKG:debian/hppa_op_optimize_workaround - https://bugs.debian.org/838613 Temporarily lower the optimization of op.c on hppa due to gcc-6 problems
    DEBPKG:debian/installman-utf8 - https://bugs.debian.org/840211 Generate man pages with UTF-8 characters
    DEBPKG:fixes/file_path_chmod_race - https://bugs.debian.org/863870 [rt.cpan.org #121951] Prevent directory chmod race attack.
    DEBPKG:fixes/extutils_file_path_compat - Correct the order of tests of chmod(). (#294)
    DEBPKG:fixes/getopt-long-2 - [rt.cpan.org #120300] Withdraw part of commit 5d9947fb445327c7299d8beb009d609bc70066c0, which tries to implement more GNU getopt_long campatibility. GNU
    DEBPKG:fixes/getopt-long-3 - provide a default value for optional arguments
    DEBPKG:fixes/getopt-long-4 - https://bugs.debian.org/864544 [rt.cpan.org #122068] Fix issue #122068.
    DEBPKG:fixes/test-builder-reset - https://bugs.debian.org/865894 Reset inside subtest maintains parent
    DEBPKG:debian/hppa_opmini_optimize_workaround - https://bugs.debian.org/869122 Lower the optimization level of opmini.c on hppa
    DEBPKG:debian/sh4_op_optimize_workaround - https://bugs.debian.org/869373 Also lower the optimization level of op.c and opmini.c on sh4
    DEBPKG:fixes/json-pp-example - [rt.cpan.org #92793] https://bugs.debian.org/871837 fix RT-92793: bug in SYNOPSIS
    DEBPKG:debian/perldoc-pager - https://bugs.debian.org/870340 [rt.cpan.org #120229] Fix perldoc terminal escapes when sensible-pager is less
    DEBPKG:debian/prune_libs - https://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need.
    DEBPKG:debian/configure-regen - https://bugs.debian.org/762638 Regenerate Configure et al. after probe unit changes
    DEBPKG:fixes/rename-filexp.U-phase1 - regen-configure: rename filexp.U to filexp_path.U, phase 1
    DEBPKG:fixes/rename-filexp.U-phase2 - regen-configure: rename filexp.U to filexp_path.U, phase 2
    DEBPKG:fixes/packaging_test_skips - Skip various tests if PERL_BUILD_PACKAGING is set
    DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian
    DEBPKG:fixes/encode-alias-regexp - https://bugs.debian.org/880085 fix https://github.com/dankogai/p5-encode/issues/127
    DEBPKG:fixes/regex-memory-leak - [910a6a8] https://bugs.debian.org/891196 [perl #132892] perl #132892: avoid leak by mortalizing temporary copy of pattern
    DEBPKG:fixes/CVE-2018-6797 - [perl #132227] (perl #132227) restart a node if we change to uni rules within the node and encounter a sharp S
    DEBPKG:fixes/CVE-2018-6798/pt1 - [perl #132063] Heap buffer overflow
    DEBPKG:fixes/CVE-2018-6798/pt2 - [perl #132063] 5.26.1: fix TRIE_READ_CHAR and DECL_TRIE_TYPE to account for non-utf8 target
    DEBPKG:fixes/CVE-2018-6798/pt3 - [perl #132063] (perl #132063) we should no longer warn for this code
    DEBPKG:fixes/CVE-2018-6798/pt4 - [perl #132063] utf8.c: Don't dump malformation past first NUL
    DEBPKG:fixes/CVE-2018-6913 - [perl #131844] (perl #131844) fix various space calculation issues in pp_pack.c
    DEBPKG:fixes/CVE-2018-12015.patch - [PATCH] [PATCH] Remove existing files before overwriting them
    DEBPKG:fixes/CVE-2018-18311.patch - [PATCH] Perl_my_setenv(); handle integer wrap
    DEBPKG:fixes/CVE-2018-18312.patch - [PATCH 242/242] PATCH: [perl #133423] for 5.26 maint
    DEBPKG:fixes/CVE-2018-18313.patch - [PATCH] regcomp.c: Convert some strchr to memchr
    DEBPKG:fixes/CVE-2018-18314.patch - [PATCH] fix #131649 - extended charclass can trigger assert
  Built under linux
  Compiled at Nov 19 2018 15:54:44
  %ENV:
    PERL5LIB="/mnt/h/dseynhae/perl5/lib/perl5"
    PERL_LOCAL_LIB_ROOT="/mnt/h/dseynhae/perl5"
    PERL_MB_OPT="--install_base "/mnt/h/dseynhae/perl5""
    PERL_MM_OPT="INSTALL_BASE=/mnt/h/dseynhae/perl5"
  @INC:
    /mnt/h/dseynhae/perl5/lib/perl5/5.26.1/x86_64-linux-gnu-thread-multi
    /mnt/h/dseynhae/perl5/lib/perl5/5.26.1
    /mnt/h/dseynhae/perl5/lib/perl5/x86_64-linux-gnu-thread-multi
    /mnt/h/dseynhae/perl5/lib/perl5
    /etc/perl
    /usr/local/lib/x86_64-linux-gnu/perl/5.26.1
    /usr/local/share/perl/5.26.1
    /usr/lib/x86_64-linux-gnu/perl5/5.26
    /usr/share/perl5
    /usr/lib/x86_64-linux-gnu/perl/5.26
    /usr/share/perl/5.26
    /mnt/h/dseynhae/perl5/lib/perl5/5.26.0
    /mnt/h/dseynhae/perl5/lib/perl5/5.26.0/x86_64-linux-gnu-thread-multi
    /usr/local/lib/site_perl
    /usr/lib/x86_64-linux-gnu/perl-base```

Perl / perl5

lc on variable with undefined content should remain undefined #18269