If you are accessing the data serially (and even if you're not) using a tied hash with MLDBM would make a good choice. The modifications would probably be simple. The biggest issue being you can't directly assign to sub elements of a complex data structure.

$MLDBM_Hash{$user}{$subkey} = $value; # Won't work.

my $hash = $MLDBM_Hash{$user};   # Will work.
$hash->{$subkey} = $value;       # Modify
$MLDBM_Hash{$user} = $hash;      # Reassign
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-Lee

"To be civilized is to deny one's nature."