open(FILEHANDLE,EXPR)

     open(FILEHANDLE)

     open FILEHANDLE

             Opens the file whose filename is given by EXPR,  and
             associates  it with FILEHANDLE.  If FILEHANDLE is an
             expression, its value is used as  the  name  of  the
             real  filehandle  wanted.   If  EXPR is omitted, the
             scalar variable of the same name as  the  FILEHANDLE
             contains  the filename.  If the filename begins with
             "<" or nothing, the file is opened  for  input.   If
             the filename begins with ">", the file is opened for
             output.  If the filename begins with ">>", the  file
             is  opened  for  appending.   (You  can put a '+' in
             front of the '>' or '<' to indicate  that  you  want
             both  read  and  write  access to the file.)  If the
             filename begins with "|",  the  filename  is  inter-
             preted  as a command to which output is to be piped,
             and if the filename ends with a "|", the filename is
             interpreted  as  command  which  pipes  input to us.
             (You may not have a command that pipes both  in  and
             out.)   Opening  '-'  opens  STDIN  and opening '>-'
             opens STDOUT.  Open returns non-zero  upon  success,
             the undefined value otherwise.  If the open involved
             a pipe, the return value happens to be  the  pid  of
             the subprocess.  Examples:

                  $article = 100;
                  open article || die "Can't find article $article: $!\n";
                  while (<article>) {...

                  open(LOG, '>>/usr/spool/news/twitlog');
                                      # (log is reserved)

                  open(article, "caesar <$article |");
                                      # decrypt article

                  open(extract, "|sort >/tmp/Tmp$$");
                                      # $$ is our process#

                  # process argument list of files along with any includes

                  foreach $file (@ARGV) {
                       do process($file, 'fh00');    # no pun intended
                  }

                  sub process {
                       local($filename, $input) = @_;
                       $input++;      # this is a string increment
                       unless (open($input, $filename)) {
                            print STDERR "Can't open $filename: $!\n";
                            return;
                       }
                       while (<$input>) {       # note use of indirection
                            if (/^#include "(.*)"/) {
                                 do process($1, $input);
                                 next;
                            }
                            ...       # whatever
                       }
                  }

             You may also, in the Bourne shell tradition, specify
             an  EXPR beginning with ">&", in which case the rest
             of the string  is  interpreted  as  the  name  of  a
             filehandle (or file descriptor, if numeric) which is
             to be duped and opened.  You may use & after >,  >>,
             <,  +>,  +>>  and  +<.   The mode you specify should
             match the mode of the original filehandle.  Here  is
             a  script that saves, redirects, and restores STDOUT
             and STDERR:


                  #!/usr/bin/perl
                  open(SAVEOUT, ">&STDOUT");
                  open(SAVEERR, ">&STDERR");

                  open(STDOUT, ">foo.out") || die "Can't redirect stdout";
                  open(STDERR, ">&STDOUT") || die "Can't dup stdout";

                  select(STDERR); $| = 1;       # make unbuffered
                  select(STDOUT); $| = 1;       # make unbuffered

                  print STDOUT "stdout 1\n";    # this works for
                  print STDERR "stderr 1\n";    # subprocesses too

                  close(STDOUT);
                  close(STDERR);

                  open(STDOUT, ">&SAVEOUT");
                  open(STDERR, ">&SAVEERR");

                  print STDOUT "stdout 2\n";
                  print STDERR "stderr 2\n";

             Explicitly closing any piped filehandle  causes  the
             parent  process to wait for the child to finish, and
             returns the status value in $?.  Note: on any opera-
             tion  which  may do a fork, unflushed buffers remain
             unflushed in both processes,  which  means  you  may
             need to set $| to avoid duplicate output.

             On MS-DOS, "pipes" are actually files, and the  sub-
             process is run to completion.  For output pipes, the
             subprocess is run when perl exits or when  the  pipe
             is  explicitly closed.  For input pipes, the subpro-
             gram is run to completion when the open is  done.
             On MS-DOS, it is recommended that forward slashes be
             used to delimit path components in file names.  (See
             the section on MS-DOS.)