How about:

>perl -wMstrict -le
"my $str = 'a b c d e 96';
 my $last_field = (split / /, $str)[-1];
 print qq{'$last_field'};
"
'96'
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... the primary question is why doesn't the following construct work:
 (@junk, $ads) = (split(/\s+/, $rec));

Because the  @junk array 'consumes' all elements of the list produced by the split built-in, leaving nothing but undefinedness for the  $ads scalar.