#!/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
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Outputs:

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
[download]