...

my ($action, $file, $key) = @ARGV;

my $input;
{
   open(my $fh, '<', $file)
      or die("Can't open $file: $!\n");
   binmode($fh);
   local $/;
   $input = <$fh>;
}

if ($action eq '-e') {
   print encrypt($input, $key);
} else {
   print decrypt($input, $key);
}