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use strict;
use warnings 'all';
...
    first  => sub { factorial_r(50) },
    second => sub { factorial_i(50) },
});

Benchmark: timing 100000 iterations of factorial_i, factorial_i_extra,
+ factorial_r, factorial_r_extra...
factorial_i:  5 wallclock secs ( 4.94 usr +  0.00 sys =  4.94 CPU) @ 2
+0251.11/s (n=100000)
factorial_i_extra:  6 wallclock secs ( 5.67 usr +  0.00 sys =  5.67 CP
+U) @ 17633.57/s (n=100000)
factorial_r:  6 wallclock secs ( 6.44 usr +  0.01 sys =  6.45 CPU) @ 1
+5496.67/s (n=100000)
factorial_r_extra: 13 wallclock secs (11.98 usr +  0.00 sys = 11.98 CP
+U) @ 8344.46/s (n=100000)