I can reproduce this on one of my machines... (Cobalt RaQ, RH 6.2, perl 5.6.1)

% perl -le 'print "($1) " while "this_is_broken_" =~ /(.*?)_/gs'
(this)

% perl -mre=debug -le 'print "($1) "while "this_is_broken_" =~ /(.*?)_
+/gs'
Freeing REx: `,'
Compiling REx `(.*?)_'
size 10 first at 5
   1: OPEN1(3)
   3:   MINMOD(4)
   4:   STAR(6)
   5:     SANY(0)
   6: CLOSE1(8)
   8: EXACT <_>(10)
  10: END(0)
floating `_' at 0..2147483647 (checking floating) anchored(SBOL) impli
+cit minlen 1 
Guessing start of match, REx `(.*?)_' against `this_is_broken_'...
Found floating substr `_' at offset 4...
Guessed: match at offset 0
Matching REx `(.*?)_' against `this_is_broken_'
  Setting an EVAL scope, savestack=9
   0 <> <this_is_brok>    |  1:  OPEN1
   0 <> <this_is_brok>    |  3:  MINMOD
   0 <> <this_is_brok>    |  4:  STAR
  Setting an EVAL scope, savestack=9
   0 <> <this_is_brok>    |  6:    CLOSE1
   0 <> <this_is_brok>    |  8:    EXACT <_>
                              failed...
                           SANY can match 1 times out of 1...
   1 <t> <his_is_brok>    |  6:    CLOSE1
   1 <t> <his_is_brok>    |  8:    EXACT <_>
                              failed...
                           SANY can match 1 times out of 1...
   2 <th> <is_is_brok>    |  6:    CLOSE1
   2 <th> <is_is_brok>    |  8:    EXACT <_>
                              failed...
                           SANY can match 1 times out of 1...
   3 <thi> <s_is_brok>    |  6:    CLOSE1
   3 <thi> <s_is_brok>    |  8:    EXACT <_>
                              failed...
                           SANY can match 1 times out of 1...
   4 <this> <_is_brok>    |  6:    CLOSE1
   4 <this> <_is_brok>    |  8:    EXACT <_>
   5 <this_> <is_brok>    | 10:    END
Match successful!
(this) 
Guessing start of match, REx `(.*?)_' against `is_broken_'...
Not at start...
Match rejected by optimizer
Freeing REx: `(.*?)_'

% perl -V
Summary of my perl5 (revision 5.0 version 6 subversion 1) configuratio
+n:
  Platform:
    osname=linux, osvers=2.2.14c11, archname=i586-linux
    uname='linux cobalt.blakem.com 2.2.14c11 #2 wed jun 28 00:55:51 pd
+t 2000 i586 unknown '
    config_args='-de'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef use5005threads=undef useithreads=undef usemultipl
+icity=undef
    useperlio=undef d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
  Compiler:
    cc='cc', ccflags ='-fno-strict-aliasing -I/usr/local/include -D_LA
+RGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2',
    cppflags='-fno-strict-aliasing -I/usr/local/include'
    ccversion='', gccversion='2.95.2 19991024 (release)', gccosandvers
+=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=1
+2
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t',
+ lseeksize=8
    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 -lndbm -lgdbm -ldb -ldl -lm -lc -lposix -lcrypt -lutil
    perllibs=-lnsl -ldl -lm -lc -lposix -lcrypt -lutil
    libc=/lib/libc-2.1.3.so, so=so, useshrplib=false, libperl=libperl.
+a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynami
+c'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'


Characteristics of this binary (from libperl): 
  Compile-time options: USE_LARGE_FILES
  Built under linux
  Compiled at Oct 30 2001 03:49:28
  %ENV:
    PERL5LIB="/web/httpd_perl/perllibs:/usr/local/perllibs"
  @INC:
    /web/httpd_perl/perllibs
    /usr/local/perllibs
    /usr/local/lib/perl5/5.6.1/i586-linux
    /usr/local/lib/perl5/5.6.1
    /usr/local/lib/perl5/site_perl/5.6.1/i586-linux
    /usr/local/lib/perl5/site_perl/5.6.1
    /usr/local/lib/perl5/site_perl
    .
[download]

-Blake