my ($min) = keys(%jetsons); This is the number of keys in %jetsons.

Actually, it's a randomly (insofar as the hashing algorithm is random) chosen key from the hash. The assignment is in list context and seeds the min/max finder loop that follows.

>perl -wMstrict -le
"my %jetsons = qw(aa 1  bb 2  cc 3  dd 4  ee 5);
 my ($min) = keys %jetsons;
 print $min;
"
cc
[download]

Update: ikegami's code works. (Never doubt a Pope.)