# Define a global variable, the compatible way
use vars qw($foo);

# Define a global variable, the Perl 5.6.x way
our $foo;

# Define a "lexical" variable
my $foo;


##</code><code>##


undef $/;             # Enables whole file mode
open (FH, $filename); # Opens the file 
while(<FH>) {         # Contains the whole file
  blah
}


##</code><code>##

{
   undef local $/;
   open (FH, $filename);
   while(<FH>) {
      blah
   }
}