Some examples :

#!/usr/bin/perl 
use strict;
use warnings;
use DateTime;

my $dt = DateTime->last_day_of_month( year => 2009, month => 2 );

while ( $dt->mday() >= 1 and $dt->month == 2 ) { 
    $dt->subtract( days => 1 );
    print "Last Friday of February: ", $dt->ymd, "\n" and last
      if $dt->day_of_week == 5;
}

my $dt1 = DateTime->new( year => 2009, month => 1, day => 1 );

while (1) {
    $dt1->add( days => 1 );
    print "First Monday of January: ", $dt1->ymd, "\n" and last
      if $dt1->day_of_week == 1;
}
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hth,
PooLpi