Perl Regex Unexpected Performance Hit

Perl v5.28.1

The benchmark :

use common::sense;
use Benchmark qw(:all);

my $UPPER = 10_000_000;
my $str = 'foo bar baz';

cmpthese(10, {
        'empty for-loop' => sub {
                        for my $i (1..$UPPER) {}
                },
        'regex match' => sub {
                        for my $i (1..$UPPER) {
                                $str =~ /foo/;
                        }
                },
        'regex match (single compile)' => sub {
                        my $re = qr/foo/;
                        for my $i (1..$UPPER) {
                                $str =~ $re;
                        }
                },
        'regex match (anchor)' => sub {
                        for my $i (1..$UPPER) {
                                $str =~ /^foo/;
                        }
                },
        'regex match (anchor) (single compile)' => sub {
                        my $re = qr/^foo/;
                        for my $i (1..$UPPER) {
                                $str =~ $re;
                        }
                },
});

The results :

                                      s/iter regex match (anchor) (single compile) regex match (single compile) regex match (anchor) regex match empty for-loop
regex match (anchor) (single compile)   3.83                                    --                         -21%                 -60%        -84%           -97%
regex match (single compile)            3.04                                   26%                           --                 -50%        -80%           -96%
regex match (anchor)                    1.53                                  151%                          99%                   --        -61%           -92%
regex match                            0.601                                  537%                         405%                 154%          --           -81%
empty for-loop                         0.117                                 3170%                        2496%                1205%        414%             --

Because foo happens to occur at the start of the string, I would expect adding an explicit anchor (^) to the regex to do nothing ... not halve performance !

As well, I've read something to the effect that Perl is smart enough to not recompile expressions with fixed strings, even when contained within loops.
But why would attempting to manually/explicitly "precompile" an expression into variable $re cause such a performance hit ?!

I changed the search substring "foo" to "asdf" (which does not occur in $str), and anchoring does let the engine drop out of searching sooner. But assigning the expression into a variable is still a massive performance hit - much more than I would have expected ! :

                                         Rate regex match (single compile) regex match (anchor) (single compile) regex match regex match (anchor) empty for-loop
regex match (single compile)          0.401/s                           --                                  -10%        -79%                 -83%           -96%
regex match (anchor) (single compile) 0.447/s                          11%                                    --        -76%                 -81%           -95%
regex match                            1.88/s                         369%                                  321%          --                 -19%           -79%
regex match (anchor)                   2.33/s                         481%                                  421%         24%                   --           -75%
empty for-loop                         9.17/s                        2185%                                 1951%        387%                 294%             --

So 2 questions to summarize :
- Why should a start-of-string anchor halve performance ?
- Why should compiling an expression (qr//) into a variable be 80% slower than using the same expression in-line ?