The null filehandle <> is special and can be used to emulate the behav-
ior of sed and awk.  Input from <> comes either from standard input, or
from  each  file  listed on the command line.  Here's how it works: the
first time <> is evaluated, the ARGV array is checked,  and  if  it  is
null,  $ARGV[0]  is  set  to  '-', which when opened gives you standard
input.  The ARGV array is then processed as a list of  filenames.   The
loop

     while (<>) {
          ...            # code for each line
     }

is equivalent to

     unshift(@ARGV, '-') if $#ARGV < $[;
     while ($ARGV = shift) {
          open(ARGV, $ARGV);
          while (<ARGV>) {
               ...       # code for each line
          }
     }

except  that it isn't as cumbersome to say.  It really does shift array
ARGV and put the current filename into variable  ARGV.   It  also  uses
filehandle  ARGV  internally.  You can modify @ARGV before the first <>
as long as you leave the first filename at the beginning of the  array.