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

while (<DATA>) {
    next if ! /^\w{4}(\.\d+){3}/;
    print;
}

__DATA__
MONDAY
MD21.2.7.8

TUESDAY # Driver Code = TD20.2.7.8
TD20.2.7.8

WEDNESDAY # Driver Code = WD20.2.7.8
WD30.2.7.8

FRIDAY # Driver Code = FD20.2.7.8
FD40.2.7.8

##</code><code>##

MD21.2.7.8
TD20.2.7.8
WD30.2.7.8
FD40.2.7.8

##</code><code>##

Applying pattern match (m//) to @t will act on scalar(@t) at ...