use Time::HiRes 'time';
    
        for my $regex (
                q{^a$|^b$},
                q{^(a|b)$},
                q{(a|b)},
                q{^a$|^b$|^c$|^d$|^e$|^f$},
                q{^(a|b|c|d|e|f)$},
                q{a|b|c|d|e|f},
        ) {
                my $start = time();
        
                for my $i (1 .. 100_000) {
                        'SOMEBIGSTRINGHERE' =~ m{$regex};
                }
        
                my $runtime = time() - $start;
                printf("%50s: %f\n", $regex, $runtime);
        }


with perl 5.8.8 -

                      ^a$|^b$: 0.101017
                      ^(a|b)$: 0.017527
                        (a|b): 0.107669
      ^a$|^b$|^c$|^d$|^e$|^f$: 0.163687
              ^(a|b|c|d|e|f)$: 0.022244
                  a|b|c|d|e|f: 0.171675

with perl 5.16.2 -

              

                       ^a$|^b$: 0.254984
                       ^(a|b)$: 0.031507
                         (a|b): 0.045713
       ^a$|^b$|^c$|^d$|^e$|^f$: 0.443303
               ^(a|b|c|d|e|f)$: 0.031506
                   a|b|c|d|e|f: 0.043478

##</code><code>##

$rex = qr/my.STRING/is;
print $rex;                 # prints (?si-xm:my.STRING)
s/$rex/foo/;