Among the excellent replies above I don't think anyone explained explicitly why you are getting the answer 1 from length, although maybe it is implied.

length takes a scalar, so it uses keys %$hashref in scalar context. That gives 3, but length converts that to text '3', which is 1 character.

For example:

use warnings;
use strict;

my $hashref = {1..20};
print scalar(keys %$hashref)."\n";
print length keys %$hashref;
[download]

Gives:

10
2
[download]

because there are 2 characters in "10".