use Benchmark qw(cmpthese);

my $end = 500_000;
cmpthese( -1,
    {
        plain => sub {
            my $total;
            for(1..$end) {
                $total += 1 / $_;
            }
            $total;
        },
        sub => sub {
            my $total;
            for(1..$end) {
                $total += reciprocal($_);
            }
            $total;
        },
        do => sub {
            my $total;
            for(1..$end) {
                $total += do { 1 / $_ };
            }
            $total;
        },
        do_var => sub {
            my $total;
            for(1..$end) {
                $total += do{ my $int = $_; 1 / $int };
            }
            $total;
        },
    }
);

sub reciprocal {
    my ($int) = @_;
    1 / $int;
}
__END__
         Rate    sub do_var  plain     do
sub    6.60/s     --   -49%   -72%   -72%
do_var 12.9/s    95%     --   -46%   -46%
plain  23.9/s   261%    85%     --     0%
do     23.9/s   261%    85%     0%     --