If you don't want to do the traditional approach of buffering and re-writing the data yourself, you can try Tie::File. A small example using your test data:

#!/usr/bin/perl -wd

use Tie::File;
tie @array, 'Tie::File', "test.txt" or die "cannot tie file\n";
($date,$data) = split( /\s+/, $array[-1] );
$data = "whatever" if $date eq "200208";
$array[-1] = "$date  $data";
untie @array;
[download]

-derby