From rmiller@rmiller1.idx.com

Created by Rich_Miller@idx.com

The statement
  my $x = 0 if $y->{Z};
usually BUT NOT ALWAYS sets $x to undef or 0. When it fails, $x appears to
be set to a value to which it was set in a previous call to the subroutine
in which this statement occurs. Is this correct behavior?

Splitting the statement into two parts
  my $x;
  $x = 0 if $y->{Z};
results in code which always works correctly.

This occurs deep within the evaluation of a large and reasonably complex
Perl data structure, using only native Perl code, which is warning-free
with 'use strict' and the -w option supplied to Perl. I have been completely
unable to show this in anything remotely resembling a simple test case.

Site configuration information for perl 5.00503:

Configured by root at Mon Aug 30 23:08:56 EDT 1999.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=linux, osvers=2.2.5-22smp, archname=i386-linux
    uname='linux porky.devel.redhat.com 2.2.5-22smp #1 smp wed jun 2 09:11:51 edt 1999 i686 unknown '
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='-O2', gccversion=egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)
    cppflags='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    ccflags ='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    stdchar='char', d_stdstdio=undef, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -ldl -lm -lc -lposix -lcrypt
    libc=, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    


@INC for perl 5.00503:
    /usr/local/bin/perl
    /home/rmiller/rel/pmake
    /usr/lib/perl5/5.00503/i386-linux
    /usr/lib/perl5/5.00503
    /usr/lib/perl5/site_perl/5.005/i386-linux
    /usr/lib/perl5/site_perl/5.005
    .


Environment for perl 5.00503:
    HOME=/home/rmiller
    LANG=en_US
    LANGUAGE (unset)
    LC_ALL=en_US
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/rmiller/bin:.
    PERL5LIB=/usr/local/bin/perl:/home/rmiller/rel/pmake
    PERL_BADLANG (unset)
    SHELL=/bin/bash

From [Unknown Contact. See original ticket]

The statement
my $x = 0 if $y->{Z};
usually BUT NOT ALWAYS sets $x to undef or 0. When it fails, $x appears to
be set to a value to which it was set in a previous call to the subroutine
in which this statement occurs. Is this correct behavior?

This is actually a quasi-known "feature". Contrast​:

  use strict;
  sub counter {
  my $value if 0;
  return ++$value;
  }
  for (1..5) {
  printf "that's %d\n", counter();
  }

With​:

  use strict;
  {
  my $value;
  sub counter {
  return ++$value;
  }
  }
  for (1..5) {
  printf "that's %d\n", counter();
  }

But the former admits no initialization, because you can't say

  my $value = 0 unless defined $value;

As it's not yet in scope. Also, the second form allows this kind of thing​:

  BEGIN {
  my $value = 0;
  sub incr { return ++$value }
  sub decr { return --$value }
  }

I assume one could/should patch perlsyn/perlsub/perlfunc if one
wishes to condone this activity. Whether one wishes such, however,
remains unclear. :-)

--tom

From [Unknown Contact. See original ticket]

Thanks much for the explanation. I used the construct
  my $x = 0 if $y->{Z}
in a only one line of code, so it's easy to fix. I have no reason to "condone
the behavior" -- but it would have been nice to get a warning (either on
subsequent use of $x, if the "my $x" wasn't executed by or that a conditional
assignment isn't allowed in this context...). But no big deal.

Perl's a wonderful tool!
-- Rich Miller