From cook@sightpath.com

Perl fails to reject this program​:

$a = 1,,,,,,,,

The superfluous commas just seem to be ignored.

The camel says these are wrong (so I'm told; p528?)​:

print $a, if s/x/y/;
print, if s/x/y/;

Yet perl accepts them.

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/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=/u/cook
    LANG=en_US
    LANGUAGE (unset)
    LC_ALL=en_US
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/local/cook/sonoma/bin:/u/cook/bin:/u/cook/site/linux/bin:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin
    PERL_BADLANG (unset)
    SHELL=/bin/bash

From @TimToady

cook@​sightpath.com writes​:
: Perl fails to reject this program​:
:
: $a = 1,,,,,,,,

It also fails to reject these​:

  $a = (1,,,,,,,,)
  $a = 1,2,3,4,5,6,7,8,9,
  $a = 1,,,,,,,,,10,

: The superfluous commas just seem to be ignored.

Yes. This is a recursive consequence of the rule that any list can end
with a comma, which is ignored. And the fact that lists don't require
parens around them unless required by precedence. So don't take the
absence of parentheses for the absence of a list.

So your example is parsing as all these sublists​:

  ($a = 1),(),(),(),(),(),(),(),()

which, after you interpolate all the null lists, is just the list

  ($a = 1)

which is, of course, no different from

  $a = 1

: The camel says these are wrong (so I'm told; p528?)​:

That's talking specifically about putting a comma between a filehandle
and subsequent arguments.

: print $a, if s/x/y/;
: print, if s/x/y/;
:
: Yet perl accepts them.

Why not? There are no filehandles there, as far as Perl is concerned.
Just a list of size 1 and a list of size 0, either of which can end
with a comma.

Actually, now that I think about it, you can't actually start a list
with a comma, so the second one is probably parsing as (print),()
rather than print((),()). In which case that's a list of size 1 as well,
the first and only element of which is the bare print operator. After all,
if you said

  print, next if s/x/y/

it would print $_ and then do a next, since next would not be considered
an argument to the print. That's why we don't allow lists to start with
a comma.

But they may certainly end with one. And since a list that ends with
a comma is a list, it can in turn end with a comma. And since a list
that ends with two commas is a list, it can in turn end with a comma.
Und so weiter...

Larry