try searching, you'll find perldoc -f  localtime
(localtime)

So you'll probably want to do something like this:

my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(t
+ime);
$year += 1900;
$mon++; # month = 0 based;

if ($mon <= 6){
    $mon += 6;
    $year -=1;
}else {
    $mon -= 6;
}

print "the date minus 6 months = \" $mday - $mon - $year \"\n";
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But if you want it more exactly, you'll have to count each day in a mont, and do a more secure substraction.