There is module on CPAN that will let you work with dates in the format that you want. Date::Manip. The following code works out the current month and then whether today is the last sunday. Hope you find it useful.

#!/usr/bin/perl -w

use strict;

use Date::Manip;

my $curmonth = &UnixDate("today","%b");

my $date1=&ParseDate("today");
my $date2=&ParseDate("last sunday in $curmonth");
my $flag=&Date_Cmp($date1,$date2);
if ($flag<0) {
           # date1 is earlier
} elsif ($flag==0) {
           # the two dates are identical
} else {
           # date2 is earlier
}
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