#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11114988
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");
my $value = 'unicorn';

my @sections = grep /$value/i, (join '', @lines) =~ /^t.*?\nT.*?\n/gms;

print join "\n", @sections; # so there is a gap between sections