friendoffriend.pl runs very quickly

Yes, but it's executed 1,000,000 times.

Just loading perl takes 3ms

$ time perl -e1

real    0m0.003s
user    0m0.004s
sys     0m0.000s
[download]

Loading a basic module triples that.

$ time perl -e'use Scalar::Util;'

real    0m0.009s
user    0m0.004s
sys     0m0.004s
[download]

For every record, you do the following actions that could be avoided if you rewrote friendoffriend.pl as a module:

• Launch a new shell
• Launch a new instance of perl
• Recompiling friendoffriend.pl
• Serialize and output the result of friendoffriend.pl
• Read and parse the result of friendoffriend.pl

If that adds up to 12ms, you'd save 3 hours and 20 minutes (12ms * 1,000,000) just by converting friendoffriend.pl into a module.

So, will this large hash significantly impact execution time?

It shouldn't. Assuming all memory access costs the same, inserting into and fetching from a Perl hash takes O(1) time. In other words, the time to do those actions does not increase as the size of the hash increases.

If you use enough memory to require swapping, that would break the assumption.

Update: Reorganised to be clearer. Added actual numbers.