Sometimes using a simpler data structure is very effective in conserving memory. For example, creating an array containing 1 million fairly simple hashes:

perl -e"
my @a; 
push @a, {
  name=>'fred',
  surname=>'bloggs',
  age=>'ancient',
  dob=>'0/0/0000',
  empno=>1234567890
} for 1..1e6; 
sleep 10"
[download]

uses 673MB of ram on my 64-bit system.

However, storing exactly the same information using strings:

perl -e"
my @a; 
push @a, join( $;, 
 name=>'fred', 
 surname=>'bloggs', 
 age=>'ancient', 
 dob=>'0/0/0000', 
 empno=>1234567890 
), 1..1e6; 
sleep 10"
[download]

Only takes 87MB.

By using strings--which are easily turned back into hashes on a case by case basis as required: my %temp = split $;, $array{ $i ];--rather than hashes, during the data accumulation phase, can often mean that you can store 6 or 7 times as much data in the same memory.

Whilst there may be some performance penalty incurred as a result of building the strings then converting them back to hashes on demand, this is often far less than the performance hit of moving to disk-based storage.