use Benchmark;

print "holli\n";
timethis (999999, sub { $_ = "fox comes and fox goes into forest"; s/(fox.+)(fox.+?forest)/${1}the $2/; });
print "borisz\n";
timethis (999999, sub { $_ = "fox comes and fox goes into forest"; s/(.*)(fox.+?forest)/$1the $2/; });
print "sh1tn\n";
timethis (999999, sub { $_ = "fox comes and fox goes into forest"; s/(?<=fox)(.+?)(fox)/$1the $2/; });
print "Roy Jonhson\n";
timethis (999999, sub { $_ = "fox comes and fox goes into forest"; s/(?=fox(?:(?!.*fox).)*forest)/the /; });

##</code><code>##

holli
timethis 999999: 10 wallclock secs ( 9.87 usr +  0.00 sys =  9.87 CPU) @ 101275.98/s (n=999999)
borisz
timethis 999999: 14 wallclock secs (11.08 usr +  0.00 sys = 11.08 CPU) @ 90268.91/s (n=999999)
sh1tn
timethis 999999: 12 wallclock secs (10.41 usr +  0.01 sys = 10.42 CPU) @ 95959.98/s (n=999999)
Roy Jonhson
timethis 999999: 10 wallclock secs (10.19 usr +  0.02 sys = 10.20 CPU) @ 98000.69/s (n=999999)