But it turns out that your approach isn't as slow as I thought it would be. It's still slower than using an array, but not terribly.

Benchmark: running inlineArrayLookup, preBuiltArrayLookup, strftimeLoo
+kup, substrLookup, each for at least 2 CPU seconds...
inlineArrayLookup:  2 wallclock secs ( 2.12 usr + -0.01 sys =  2.11 CP
+U) @ 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 inlineArrayLo
+okup 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%                  --
[download]

#!/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)[getMont
+hNumber()];
}

sub substrLookup {
    return substr('JanFebMarAprMayJunJulAugSepOctNovDec',getMonthNumbe
+r()*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,
         }
     );
[download]

Edit: Added strftime variant which I had overlooked.

Edit by tye, change PRE to CODE around long lines