# Change this line if the number of entries changes.
use constant ENTRIES_PER_LINE => 4;
...
my @entries = split /\s+/, $str;
if (@entries != ENTRIES_PER_LINE) {
  warn "Wrong number of entries: '$str' at line $.\n";
}
else {
  # And update this one as well. 
  my($foo, $bar, $baz, $quux) = @entries;
  # continue with your code
}