use strict;
use warnings;

my @lines = (
    "t13:45\n", "D13:45\n", "S13:45 Unicorn\n", "D13:45\n", "S13:45\n", "T13:45\n",
    "t13:45\n", "D13:45\n", "T13:46\n",
    "t13:45\n", "D13:45\n", "S13:45\n","D13:45\n", "S13:45 UNICORN\n", "T13:45\n",
    "t13:45\n", "D13:45\n", "T13:46\n",
    );

use constant GOOD_LINE   => qr{ \A [tTJSD] }xms;
use constant END_SECTION => qr{ \A   T     }xms;

my $value = "uNiCoRn";
my @section;

LINE:
while (my $line = shift @lines) {

    die "bad line '$line'" unless $line =~ GOOD_LINE;

    push @section, $line;

    next LINE unless $line =~ END_SECTION;

    if (grep m{ (?i) \Q$value\E }xms, @section) {
        print for @section;
        print "\n";
        }

    @section = ();

    }

__END__
t13:45
D13:45
S13:45 Unicorn
D13:45
S13:45
T13:45

t13:45
D13:45
S13:45
D13:45
S13:45 UNICORN
T13:45