So the problem is that you don't know how to read a file?

my $file;
{
   open(my $fh, '<', ...) or die ...;
   local $/;
   $file = <$fh>;
}
[download]

But it also seems that your grammar isn't complete. You've defined some basic blocks, but not how they fit together. If you just want a tokenizer, you can use something like

use strict;
use warnings;

use Parse::RecDescent;

$::RD_ERRORS = 1;
$::RD_WARN   = 1;
$::RD_HINT   = 1;

my $grammar = <<'__EOI__';

    {
        use strict;
        use warnings;
    }

    tokenize : token(s?) /\Z/ { $item[1] }

    token    : ident
             | binops
             | lbinops
             | number
             | integer
             | /./ { [ skip => $items[1] ] }

    ident    : /[a-zA-Z][a-zA-Z0-9_]*/   { [ @items ] }
    binops   : /\*\*|[+\-\/*%]/          { [ @items ] }
    lbinops  : />=?|<=?|!|&&|\|\||==/    { [ @items ] }
    number   : /-?[0-9]*\.[0-9]+/        { [ @items ] }
    integer  : /-?[0-9]/                 { [ @items ] }

__EOI__

...

my @tokens = $parser->tokenize($file);
[download]