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


my $file = <<FILE;
t13:45
D13:45
S13:45 Unicorn
D13:45
S13:45
T13:45
t13:45
D13:45
T13:46
t13:45
D13:45
S13:45
D13:45
S13:45 UNICORN
T13:45
t13:45
D13:45
T13:46
FILE

my $value = 'unicorn';

open my $FH, '<', \$file or die "Cannot open 'file' because: $!";

my $section = '';
while ( <$FH> ) {
    if ( /^t/ .. /^T/ ) {
        $section .= $_;
        }
    if ( /^T/ ) {
        print $section if $section =~ /$value/i;
        $section = '';
        }
    }