From @cpansprout

Just look at this, from mg.c​:Perl_magic_set​:

  case '^'​:
  Safefree(IoTOP_NAME(GvIOp(PL_defoutgv)));
  s = IoTOP_NAME(GvIOp(PL_defoutgv)) = savesvpv(sv);
  IoTOP_GV(GvIOp(PL_defoutgv)) = gv_fetchsv(sv, GV_ADD, SVt_PVIO);
  break;
  case '~'​:
  Safefree(IoFMT_NAME(GvIOp(PL_defoutgv)));
  s = IoFMT_NAME(GvIOp(PL_defoutgv)) = savesvpv(sv);
  IoFMT_GV(GvIOp(PL_defoutgv)) = gv_fetchsv(sv, GV_ADD, SVt_PVIO);
  break;

$ ./perl -Ilib -e 'format =' -efoo -e. -e '$^=23;delete $​::{23}; $==0; write'
Assertion failed​: (SvTYPE(_gvgp) == SVt_PVGV || SvTYPE(_gvgp) == SVt_PVLV), function Perl_pp_leavewrite, file pp_sys.c, line 1475.
Abort trap​: 6

Flags​:
  category=core
  severity=low

Site configuration information for perl 5.19.4​:

Configured by sprout at Mon Sep 9 00​:16​:24 PDT 2013.

Summary of my perl5 (revision 5 version 19 subversion 4) configuration​:
  Commit id​: d47819ff6f55bfaa4b7eddc66c6db7d7dfdec11c
  Platform​:
  osname=darwin, osvers=12.2.0, archname=darwin-2level
  uname='darwin pint.local 12.2.0 darwin kernel version 12.2.0​: sat aug 25 00​:48​:52 pdt 2012; root​:xnu-2050.18.24~1release_x86_64 x86_64 '
  config_args='-de -Dcc=gcc -Dusedevel -Doptimize=-ggdb3 -Aoptimize=-O0 -DDEBUGGING'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=undef, usemultiplicity=undef
  useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
  use64bitint=define, use64bitall=define, uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='gcc', ccflags ='-fno-common -DPERL_DARWIN -no-cpp-precomp -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include',
  optimize='-ggdb3 -O0',
  cppflags='-no-cpp-precomp -fno-common -DPERL_DARWIN -no-cpp-precomp -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
  ccversion='', gccversion='4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)', 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='env MACOSX_DEPLOYMENT_TARGET=10.3 cc', ldflags =' -fstack-protector -L/usr/local/lib'
  libpth=/usr/local/lib /usr/lib
  libs=-ldbm -ldl -lm -lutil -lc
  perllibs=-ldl -lm -lutil -lc
  libc=, so=dylib, useshrplib=false, libperl=libperl.a
  gnulibc_version=''
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=bundle, d_dlsymun=undef, ccdlflags=' '
  cccdlflags=' ', lddlflags=' -bundle -undefined dynamic_lookup -L/usr/local/lib -fstack-protector'

@​INC for perl 5.19.4​:
  lib
  /usr/local/lib/perl5/site_perl/5.19.4/darwin-2level
  /usr/local/lib/perl5/site_perl/5.19.4
  /usr/local/lib/perl5/5.19.4/darwin-2level
  /usr/local/lib/perl5/5.19.4
  /usr/local/lib/perl5/site_perl
  .

Environment for perl 5.19.4​:
  DYLD_LIBRARY_PATH (unset)
  HOME=/Users/sprout
  LANG=en_US.UTF-8
  LANGUAGE (unset)
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)
  PATH=/usr/bin​:/bin​:/usr/sbin​:/sbin​:/usr/local/bin​:/usr/local/bin
  PERL_BADLANG (unset)
  SHELL=/bin/bash