I tend to not use the sub variant, but the string variant. Using a sub means entering and leaving a scope, and that takes time. If that time is significant compared to what you are benchmarking, your results are far less useful than they could be. Watch:

#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw /cmpthese/;

our ($x, $y);

cmpthese -1  => {
    sub      => sub {$::x ++},
    string   =>     '$::y ++',
}

__END__
             Rate    sub string
sub     9175039/s     --   -29%
string 12862205/s    40%     --
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Abigail