You probably have blank lines in your input.

In the split doco, under "As another special case, ...", you'll see how your pattern (' ') is handled specially.

Whenever $line equals "\n", split(' ',$line) will return (): all of those 10 variables ($site, ..., $length) will be undef (i.e. "uninitialized value").

If you want to ignore them, you can do something like this:

foreach my $line (<LOGFILE>) {
    next if $line =~ /\A\Z/;
    ($site, ...
    ...
}
[download]

[Aside: $a and $b are not good choices for variable names as they have special meaning to Perl - see perlvar]