I can't answer your question, however it is interesting to consider the two cases:

use strict;
use warnings;
use Benchmark qw(cmpthese);

my $HASHSIZE = 1e3;
my %hash;

test ();

@hash{1..$HASHSIZE} = () x $HASHSIZE;

test ();

sub test {
    my $_keys = keys %hash;
    my $_scalar_keys = scalar(keys %hash);
    my $_scalar = scalar(%hash);
    my $_if = %hash;
    
    print "_keys: $_keys\n";
    print "_scalar_keys: $_scalar_keys\n";
    print "_scalar: $_scalar\n";
    print "_if: $_if\n";
    
    cmpthese(-1,
              {
               _keys => sub {if (keys %hash) { ; }},
               _scalar_keys => sub {if (scalar(keys %hash)) { ; }},
               _scalar => sub {if (scalar(%hash)) { ; }},
               _if => sub {if (%hash) { ; }},
              }
            );
}
[download]

Prints:

_keys: 0
_scalar_keys: 0
_scalar: 0
_if: 0
                  Rate      _scalar          _if        _keys _scalar_
+keys
_scalar      5217686/s           --         -10%         -15%         
+-18%
_if          5766838/s          11%           --          -6%         
+-10%
_keys        6136894/s          18%           6%           --         
+ -4%
_scalar_keys 6387143/s          22%          11%           4%         
+  --
_keys: 1000
_scalar_keys: 1000
_scalar: 629/1024
_if: 629/1024
                  Rate      _scalar          _if        _keys _scalar_
+keys
_scalar       379905/s           --          -1%         -92%         
+-92%
_if           384350/s           1%           --         -92%         
+-92%
_keys        4930608/s        1198%        1183%           --         
+ -1%
_scalar_keys 4996935/s        1215%        1200%           1%         
+  --
[download]

Note that I prefer to use cmpthese and that unless really huge numbers of tests are made without anything much else going on, the time is sufficiently small as to not matter anyway.