my $date = `date`;
my @cdate = split(/ /, $date);
my $cmonth = mapmonth($cdate[1]);
my $delstart = ($cmonth - 2);
my $delstop = ($cmonth + 1);
my @range = ($delstop .. $delstart);

[...]

if ($monthnr != @range)  {
open(NEWWTMP,">>$newwtmp");
print NEWWTMP $_;
}