Compound statements

     In perl, a sequence of commands may be treated as  one  com-
     mand by enclosing it in curly brackets.  We will call this a
     BLOCK.

     The following compound commands may be used to control flow:

          if (EXPR) BLOCK
          if (EXPR) BLOCK else BLOCK
          if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
          LABEL while (EXPR) BLOCK
          LABEL while (EXPR) BLOCK continue BLOCK
          LABEL for (EXPR; EXPR; EXPR) BLOCK
          LABEL foreach VAR (ARRAY) BLOCK
          LABEL BLOCK continue BLOCK

     Note that, unlike C and Pascal, these are defined  in  terms
     of BLOCKs, not statements.  This means that the curly brack-
     ets are required--no dangling statements  allowed.   If  you
     want  to write conditionals without curly brackets there are
     several other ways to do it.  The following all do the  same
     thing:

          if (!open(foo)) { die "Can't open $foo: $!"; }
          die "Can't open $foo: $!" unless open(foo);
          open(foo) || die "Can't open $foo: $!"; # foo or bust!
          open(foo) ? 'hi mom' : die "Can't open $foo: $!";
                         # a bit exotic, that last one


     The if  statement  is  straightforward.   Since  BLOCKs  are
     always  bounded  by curly brackets, there is never any ambi-
     guity about which if an else goes with.  If you  use  unless
     in place of if, the sense of the test is reversed.

     The while statement  executes  the  block  as  long  as  the
     expression  is true (does not evaluate to the null string or
     0).  The LABEL is optional, and if present, consists  of  an
     identifier  followed  by  a colon.  The LABEL identifies the
     loop for the loop control statements next,  last,  and  redo
     (see  below).   If  there  is a continue BLOCK, it is always
     executed  just  before  the  conditional  is  about  to   be
     evaluated  again,  similarly to the third part of a for loop
     in C.  Thus it can be used to  increment  a  loop  variable,
     even when the loop has been continued via the next statement
     (similar to the C "continue" statement).

     If the word while is replaced by the word until,  the  sense
     of the test is reversed, but the conditional is still tested
     before the first iteration.

     In either the if or the while  statement,  you  may  replace
     "(EXPR)"  with  a  BLOCK, and the conditional is true if the
     value of the last command in that block is true.

     The for loop works  exactly  like  the  corresponding  while
     loop:

          for ($i = 1; $i < 10; $i++) {
               ...
          }

     is the same as

          $i = 1;
          while ($i < 10) {
               ...
          } continue {
               $i++;
          }


     The foreach loop iterates over a normal array value and sets
     the  variable  VAR  to be each element of the array in turn.
     The variable is implicitly local to the  loop,  and  regains
     its  former value upon exiting the loop.  The "foreach" key-
     word is actually identical to the "for" keyword, so you  can
     use  "foreach" for readability or "for" for brevity.  If VAR
     is omitted, $_ is set to each value.  If ARRAY is an  actual
     array  (as  opposed  to  an  expression  returning  an array
     value), you can modify each element of the array by  modify-
     ing VAR inside the loop.  Examples:

          for (@ary) { s/foo/bar/; }

          foreach $elem (@elements) {
               $elem *= 2;
          }

          for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
               print $_, "\n"; sleep(1);
          }

          for (1..15) { print "Merry Christmas\n"; }

          foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
               print "Item: $item\n";
          }


     The BLOCK by itself (labeled or not) is equivalent to a loop
     that  executes  once.  Thus you can use any of the loop con-
     trol statements in it to leave or restart  the  block.   The
     continue  block is optional.  This construct is particularly
     nice for doing case structures.

          foo: {
               if (/^abc/) { $abc = 1; last foo; }
               if (/^def/) { $def = 1; last foo; }
               if (/^xyz/) { $xyz = 1; last foo; }
               $nothing = 1;
          }

     There is no official switch statement in perl, because there
     are  already several ways to write the equivalent.  In addi-
     tion to the above, you could write

          foo: {
               $abc = 1, last foo  if /^abc/;
               $def = 1, last foo  if /^def/;
               $xyz = 1, last foo  if /^xyz/;
               $nothing = 1;
          }

     or

          foo: {
               /^abc/ && do { $abc = 1; last foo; };
               /^def/ && do { $def = 1; last foo; };
               /^xyz/ && do { $xyz = 1; last foo; };
               $nothing = 1;
          }

     or

          foo: {
               /^abc/ && ($abc = 1, last foo);
               /^def/ && ($def = 1, last foo);
               /^xyz/ && ($xyz = 1, last foo);
               $nothing = 1;
          }

     or even

          if (/^abc/)
               { $abc = 1; }
          elsif (/^def/)
               { $def = 1; }
          elsif (/^xyz/)
               { $xyz = 1; }
          else
               {$nothing = 1;}

     As it happens, these  are  all  optimized  internally  to  a
     switch  structure,  so  perl  jumps  directly to the desired
     statement, and you needn't worry about perl executing a  lot
     of  unnecessary  statements  when  you  have  a string of 50
     elsifs, as long as you are testing the  same  simple  scalar
     variable  using  ==,  eq, or pattern matching as above.  (If
     you're curious as to whether the optimizer has done this for
     a  particular  case statement, you can use the -D1024 switch
     to list the syntax tree before execution.)