From @cpansprout

$ perl -le' @​a = (1,2,3); print shift @​a for @​a'
1
2

$ perl -le' @​a = (1,2,3); print shift @​a for (),@​a'
1
2
3

There seems to be some optimisation for ‘for @​array’ that has strange
side-effects.

This can sometimes cause action-at-a-distance​:
$node = ... expr returning an XML​::DOM​::Lite​::Node
$node->removeChild($_) for @​{ $node->childNodes };

And, apparently, I’m not the only person to have run into this. I
stumbled across this In Silki​::Web​::Form 0.08 today​:

sub _collapse_single_option_selects {
  my $self = shift;

  my @​to_collapse;
  for my $select ( @​{ $self->_dom()->getElementsByTagName
('select') } ) {
  next if $select->id() =~ /^wpms-/;

  my @​options = $select->options();

  next if @​options != 1;

  push @​to_collapse, [ $select, $options[0] ];
  }

  # Modifying the dom as we loop through it seems to cause weirdness
  # where some select elements get skipped.
  $self->_collapse_single_option_select( @​{$_} ) for @​to_collapse;
}

See that comment. A simple ‘for my $select( (), @​{...} )’ would have
worked around the problem.

Use of uninitialized value $category in concatenation (.) or string
at /usr/local/bin/perlbug5.13.1 line 645.
Use of uninitialized value $severity in concatenation (.) or string
at /usr/local/bin/perlbug5.13.1 line 645.

Flags​:
  category=core
  severity=low

Site configuration information for perl 5.13.1​:

Configured by sprout at Sun Jun 6 14​:31​:27 PDT 2010.

Summary of my perl5 (revision 5 version 13 subversion 1 patch
v5.13.1-149-g6dd2be5) configuration​:
  Snapshot of​: 6dd2be5
  Platform​:
  osname=darwin, osvers=10.0.0, archname=darwin-2level
  uname='darwin pint.local 10.0.0 darwin kernel version 10.0.0​: fri
jul 31 22​:47​:34 pdt 2009; root​:xnu-1456.1.25~1release_i386 i386 '
  config_args='-de -Dusedevel'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=undef, usemultiplicity=undef
  useperlio=define, d_sfio=undef, uselargefiles=define,
usesocks=undef
  use64bitint=undef, use64bitall=undef, uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='cc', ccflags ='-fno-common -DPERL_DARWIN -no-cpp-precomp -fno-
strict-aliasing -pipe -fstack-protector -I/usr/local/include',
  optimize='-O3',
  cppflags='-no-cpp-precomp -fno-common -DPERL_DARWIN -no-cpp-
precomp -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/
include'
  ccversion='', gccversion='4.2.1 (Apple Inc. build 5646)',
gccosandvers=''
  intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
  ivtype='long', ivsize=4, 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=/usr/lib/libc.dylib, 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'

Locally applied patches​:

@​INC for perl 5.13.1​:
  /usr/local/lib/perl5/site_perl/5.13.1/darwin-2level
  /usr/local/lib/perl5/site_perl/5.13.1
  /usr/local/lib/perl5/5.13.1/darwin-2level
  /usr/local/lib/perl5/5.13.1
  /usr/local/lib/perl5/site_perl
  .

Environment for perl 5.13.1​:
  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/X11/bin​:/
usr/local/bin
  PERL_BADLANG (unset)
  SHELL=/bin/bash

From @iabyn

On Sun, Jun 13, 2010 at 06​:23​:30PM -0700, Father Chrysostomos wrote​:

$ perl -le' @​a = (1,2,3); print shift @​a for @​a'
1
2

$ perl -le' @​a = (1,2,3); print shift @​a for (),@​a'
1
2
3

There seems to be some optimisation for ‘for @​array’ that has strange
side-effects.

The first one iterates over the elements of an array, the second over the
elements of a list. The array is being modified mid-iteration, while the
list isn't, which is why you see different results.

--
Atheism is a religion like not collecting stamps is a hobby

The RT System itself - Status changed from 'new' to 'open'

From @cowens

On Sun, Jun 13, 2010 at 21​:23, Father Chrysostomos
<perlbug-followup@​perl.org> wrote​:
snip

$ perl -le' @​a = (1,2,3); print shift @​a for @​a'
1
2
snip

This a documented as a bad thing to do in [perlsyn][1]​:

If any part of LIST is an array, foreach will get very confused if you
add or remove elements within the loop body, for example with splice.
So don't do that.

[1] : http​://perldoc.perl.org/perlsyn.html#Foreach-Loops

--
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.

@iabyn - Status changed from 'open' to 'stalled'