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     Packages

     Perl provides a mechanism for alternate namespaces  to  pro-
     tect  packages  from  stomping on each others variables.  By
     default, a perl script starts  compiling  into  the  package
     known as "main".  By use of the package declaration, you can
     switch namespaces.  The scope of the package declaration  is
     from  the  declaration  itself  to  the end of the enclosing
     block (the same scope as the local()  operator).   Typically
     it  would  be the first declaration in a file to be included
     by the "require" operator.  You can switch into a package in
     more than one place; it merely influences which symbol table
     is used by the compiler for the rest of that block.  You can
     refer to variables and filehandles in other packages by pre-
     fixing the identifier with the package  name  and  a  single
     quote.   If  the package name is null, the "main" package as
     assumed.

     Only identifiers starting with letters  are  stored  in  the
     packages  symbol table.  All other symbols are kept in pack-
     age "main".  In addition,  the  identifiers  STDIN,  STDOUT,
     STDERR,  ARGV, ARGVOUT, ENV, INC and SIG are forced to be in
     package "main", even when used for other purposes than their
     built-in  one.  Note also that, if you have a package called
     "m", "s" or "y", the you can't use the qualified form of  an
     identifier since it will be interpreted instead as a pattern
     match, a substitution or a translation.

     Eval'ed strings are compiled in the  package  in  which  the
     eval  was  compiled  in.   (Assignments  to $SIG{}, however,
     assume the signal handler specified is in the main  package.
     Qualify the signal handler name if you wish to have a signal
     handler in a package.)  For an example, examine perldb.pl in
     the  perl  library.  It initially switches to the DB package
     so that the debugger doesn't interfere with variables in the
     script you are trying to debug.  At various points, however,
     it temporarily switches back to the main package to evaluate
     various expressions in the context of the main package.

     The symbol table for a package happens to be stored  in  the
     associative array of that name prepended with an underscore.
     The value in each entry of the associative array is what you
     are  referring to when you use the *name notation.  In fact,
     the following have the same effect (in  package  main,  any-
     way), though the first is more efficient because it does the
     symbol table lookups at compile time:

          local(*foo) = *bar;
          local($_main{'foo'}) = $_main{'bar'};

     You can use this to print out all the variables in  a  pack-
     age,  for  instance.   Here  is  dumpvar.pl  from  the  perl
     library:

          package dumpvar;

          sub main'dumpvar {
              ($package) = @_;
              local(*stab) = eval("*_$package");
              while (($key,$val) = each(%stab)) {
                  {
                      local(*entry) = $val;
                      if (defined $entry) {
                          print "\$$key = '$entry'\n";
                      }
                      if (defined @entry) {
                          print "\@$key = (\n";
                          foreach $num ($[ .. $#entry) {
                              print "  $num\t'",$entry[$num],"'\n";
                          }
                          print ")\n";
                      }
                      if ($key ne "_$package" && defined %entry) {
                          print "\%$key = (\n";
                          foreach $key (sort keys(%entry)) {
                              print "  $key\t'",$entry{$key},"'\n";
                          }
                          print ")\n";
                      }
                  }
              }
          }

     Note that, even though the subroutine is compiled in package
     dumpvar, the name of the subroutine is qualified so that its
     name is inserted into package "main".