Benchmark: running inlineArrayLookup, preBuiltArrayLookup, strftimeLookup, substrLookup, each for at least 2 CPU seconds...
inlineArrayLookup:  2 wallclock secs ( 2.12 usr + -0.01 sys =  2.11 CPU) @ 543540.28/s (n=1146870)
preBuiltArrayLookup:  1 wallclock secs ( 2.04 usr + -0.01 sys =  2.03 CPU) @ 588310.34/s (n=1194270)
strftimeLookup:  3 wallclock secs ( 1.76 usr +  0.37 sys =  2.13 CPU) @ 216666.20/s (n=461499)
substrLookup:  2 wallclock secs ( 2.09 usr +  0.00 sys =  2.09 CPU) @ 465374.16/s (n=972632)
                        Rate strftimeLookup substrLookup inlineArrayLookup preBuiltArrayLookup
strftimeLookup      216666/s             --         -53%              -60%                -63%
substrLookup        465374/s           115%           --              -14%                -21%
inlineArrayLookup   543540/s           151%          17%                --                 -8%
preBuiltArrayLookup 588310/s           172%          26%                8%                  --

##</code><code>##

#!/usr/bin/perl -w

use strict;
use Benchmark qw(cmpthese);
use POSIX 'strftime';

my @months=qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);

sub getMonthNumber {
    return int(rand(12));
}

sub preBuiltArrayLookup {
    return $months[getMonthNumber()];
}

sub inlineArrayLookup {
    return qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec)[getMonthNumber()];
}

sub substrLookup {
    return substr('JanFebMarAprMayJunJulAugSepOctNovDec',getMonthNumber()*3,3);
}

# so as to not penalize strftime with localtime call
my @fixedLocaltime=localtime;
sub strftimeLookup {
    return strftime "%m",@fixedLocaltime;
}

cmpthese(-2,
        {
             'preBuiltArrayLookup' => \&preBuiltArrayLookup,
             'inlineArrayLookup' => \&inlineArrayLookup,
             'substrLookup'   => \&substrLookup,
             'strftimeLookup'   => \&strftimeLookup,
         }
     );