Thanks for the suggestions I got so far. I'm doing this in memory to clean up the data (nice and fast) before dumping it to a database. Here are some timing results for a real example with $i = 0..21448 and $j 0..91 on an Athlon 1.2GHz with 1GB RAM. Timing is for creating the array (about 5 sec) and the rest of the time goes into garbage collection. Total size of this process is about 112 MB, so about 10% of machine RAM. CPU usage is about 99+ percent and no swapping. (This is a small example that I can test in finite time :-( )

undef @data;
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This takes 2 min 56 sec Now do

for (my $i=0;$i<@data;$i++)
{
   undef @{$data[$i]);
}
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This takes 1 min 39 sec Finally do

for (my $i=0;$i<@data;$i++)
{
   $data[$i] = ();
}
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This takes 43 seconds. Adding  @data =() to the end of the last example really makes no difference performance wise.