Yes, 2M. I can't store all data in memory.

Why not? A hash containing 2 million keys takes around 65 MB of memory:

undef $h{ 0+ int rand 2**32 } for 1 .. 2e6;;
print scalar keys %h;;
1999519

print total_size \%h;;
67353411
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assuming that i need to write/search through this data rapidly from several threads

Can you say a little more about what you are doing with this data?

What would you be writing to it?

Do changes need to persist beyond the life of the program?

How many threads?

How rapidly?

I just need to check presence of certain digits in this db.

Do you mean numbers rather than "digits"?