Premature optimization is the root of all evil.
— Donald E. Knuth

As for your memory concern: the best thing to do is just try it. In almost all cases you'll find it won't be a problem.
I made it a bit shorter for you, though: 

# untested
%inter = 
    map { $_ => ($hist1{$_} <= $hist2{$_} ? $hist1{$_} : $hist2{$_}) }
    grep { $hist2{$_} } keys %hist1;

[download]

Should you run into problems, it's probably less of a headache to throw more memory at it. Purchase it for your PC or server, or rent from one of the popular "cloud" providers that charge only several cents an hour for quite a bit of memory. 

-Thomas

"Excuse me for butting in, but I'm interrupt-driven..."

In reply to Re: memory:: grep vs looping through hash by thomas895
in thread memory:: grep vs looping through hash by f77coder

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