I assume you are aware, (and it probably should be mentioned), that the referenced method is (I assume), a joke along the same lines as the included method of finding tomorrow's date:

sub tomorrow_date {
    sleep 86_400; 
    return localtime();
}
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Without all the compile-time reductions, lazy evaluation etc. that most properly functional languages benefit from, this method of determining the eveness of a number is ludicrously inefficient--6000+ times slower than the normal method:

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];
no warnings 'recursion';

sub odd { my $number = shift; return !even ($number); }
sub even { my $number = abs shift; return 1 if $number == 0; return od
+d ($number - 1); }

cmpthese -5, {
    normal     => q[ my $n;     $_ & 1 and $n++ for 1 .. 1000; ],
    functional => q[ my $n; even( $_ ) and $n++ for 1 .. 1000; ],
};

__END__
c:\test>junk
              s/iter functional     normal
functional      1.50         --      -100%
normal     2.41e-004    625042%         --
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