Inspired by BrowserUk's clever solution -- how about this ?
use strict;
use warnings;
use Time::Local;
use Test::More "no_plan";
sub lastq_bday {
my( $y, $m, $d ) = unpack 'A4A2A2', shift;
my $q = int( ($m-1) /3 ) * 3;
my $first = timelocal(0,0,12,1,$q,$y);
$first -= 60*60*24; # last day of previous quarter
my @t = localtime($first);
$t[3] -= 2 if $t[6] == 0; #Sunday
$t[3] -= 1 if $t[6] == 6; #Saturday
return sprintf ("%d%02d%02d",1900+$t[5],$t[4]+1,$t[3]);
}
is ( lastq_bday("20110901"),"20110630");
is ( lastq_bday("20120101"),"20111230");
is ( lastq_bday("20110215"),"20101231");
is ( lastq_bday("20110407"),"20110331");
is ( lastq_bday("20110808"),"20110630");
is ( lastq_bday("20110101"),"20101231");