I ran into this problem just this morning, and it's got me very curious.

The following short program, instead of triggering a syntax error, causes my perl (AS 5.8.4 on WinXP, details below) to hang up. 

#!perl
$rec = {
    avg_outst => pfo...     # BAD
    pct_pfo   => ' ',
};

[download]
By "hang up" I mean it never returns: task manager reports perl.exe using 99% of the cpu, and eating up more and more memory.

By way of explanation, this is part of a larger program I was writing, with several other fields in the hash, way down in a sub. I'd left the "pfo..." bit as a reminder to myself to come back and pull out what I wanted from the $pfo structure. Imagine my surprise when my program wouldn't even compile: not even perl -c would work.

I found the errant code by commenting out parts until I found the error, and then the fix was easy. But I'm still very curious why it happened. I went back and ripped out everything else to make the smallest test case I could, and I'm pretty sure this is it. If I remove the next line (the one starting pct_pfo =>), perl reports the syntax error and stops, as you might expect. BTW, the original was using strict and warnings.

Am I missing something obvious? This is just a plain syntax error, not some obscure abbreviation for while (1), correct? :-) Why does it completely tie up perl?

Installation details follow. I'd be curious if it happens on other platforms. 

Summary of my perl5 (revision 5 version 8 subversion 4) configuration:
  Platform:
    osname=MSWin32, osvers=4.0, archname=MSWin32-x86-multi-thread
    uname=''
    config_args='undef'
    hint=recommended, useposix=true, d_sigaction=undef
    usethreads=undef use5005threads=undef useithreads=define usemultip
+licity=define
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cl', ccflags ='-nologo -Gf -W3 -MD -Zi -DNDEBUG -O1 -DWIN32 -D
+_CONSOLE -DNO_STRICT -DHAVE_DES_FCRYPT  -DNO_HASH_SEED -DPERL_IMPLICI
+T_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO -DPERL_MSVCRT_READFIX',
    optimize='-MD -Zi -DNDEBUG -O1',
    cppflags='-DWIN32'
    ccversion='', gccversion='', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=undef, longlongsize=8, d_longdbl=define, longdblsize=10
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='__int64
+', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='link', ldflags ='-nologo -nodefaultlib -debug -opt:ref,icf  -l
+ibpath:"D:\Perl\lib\CORE"  -machine:x86'
    libpth=C:\PROGRA~1\MICROS~3\VC98\lib
    libs=  oldnames.lib kernel32.lib user32.lib gdi32.lib winspool.lib
+  comdlg32.lib advapi32.lib shell32.lib ole32.lib oleaut32.lib  netap
+i32.lib uuid.lib wsock32.lib mpr.lib winmm.lib  version.lib odbc32.li
+b odbccp32.lib msvcrt.lib
    perllibs=  oldnames.lib kernel32.lib user32.lib gdi32.lib winspool
+.lib  comdlg32.lib advapi32.lib shell32.lib ole32.lib oleaut32.lib  n
+etapi32.lib uuid.lib
wsock32.lib mpr.lib winmm.lib  version.lib odbc32.lib odbccp32.lib msv
+crt.lib
    libc=msvcrt.lib, so=dll, useshrplib=yes, libperl=perl58.lib
    gnulibc_version='undef'
  Dynamic Linking:
    dlsrc=dl_win32.xs, dlext=dll, d_dlsymun=undef, ccdlflags=' '
    cccdlflags=' ', lddlflags='-dll -nologo -nodefaultlib -debug -opt:
+ref,icf  -libpath:"D:\Perl\lib\CORE"  -machine:x86'


Characteristics of this binary (from libperl):
  Compile-time options: MULTIPLICITY USE_ITHREADS USE_LARGE_FILES PERL
+_IMPLICIT_CONTEXT PERL_IMPLICIT_SYS
  Locally applied patches:
        ActivePerl Build 810
        22751 Update to Test.pm 1.25
        21540 Fix backward-compatibility issues in if.pm
  Built under MSWin32
  Compiled at Jun  1 2004 11:52:21
  %ENV:
    PERL5LIB="F:\oracle\product\10.1.0\Db_1\sysman\admin\scripts"
  @INC:
    F:\oracle\product\10.1.0\Db_1\sysman\admin\scripts
    D:/Perl/lib
    D:/Perl/site/lib
    .

[download]

Update:
It's not a syntax error. The ... is a range operator, which perl tries to process at compile time when (both?) ends of the list are constants. That's probably good when the list is small, but is potentially disastrous when the list ends are very far from each other. Even warnings can't get a hold of it in time to say, "Possibly enormous list at line 3" or some such. So it looks like it hung up.

Do not rebuke them with harsh words ... but rather lead them gently - with URLs - so that they may learn wisdom.

In reply to Syntax error makes perl hang by VSarkiss

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