Since I'm feeling generous and it's late on a Friday afternoon so nobody is touching prod, here's a tidy-up and fix for you. The steps were as follows:

• Add strict and warnings. Just because this was originally a section of longer code doesn't mean you cannot or should not use them here.
• Fix up any contraventions of strict arising.
• Remove unreferenced variables.
• Shorten the code with postfix loops and de-duplicated elif branches. Simply the regexen while there.
• Run it through perltidy to fix the indenting.
• At this point it's clear where the problem is. Apply a simple fix (which is basically a version of choroba's solution).

#!/usr/bin/perl
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'
);
my $value      = "unicorn";
my $newsection = 0;
my $debug      = 0; # Set to 1 for verbosity
my @section;

print "Let's start\n" if $debug;
while (my $row = shift (@lines)) {

    if ($newsection <= 0) {
        if ($row =~ /^t/) {
            print "New section started\n"       if $debug;
            print "Inserting $row into array\n" if $debug;
            push (@section, $row);
        } elsif ($row =~ /^[JSD]/) {
            print "Section continued\n" if $debug;
            push (@section, $row);
        } elsif ($row =~ /^T/) {
            print "Section Ended\n" if $debug;
            push (@section, $row);
            $newsection = 1;
        }
    }
    if ($newsection > 0) {
        $newsection = 0;
        print "Checking for value\n" if $debug;
        if (grep (/$value/i, @section)) {
            print "Value discovered, saving section\n" if $debug;
            print "$_\n" for @section;
        }
        @section = ();
    }
}
[download]

Running this we get:

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
S13:45\n
D13:45\n
S13:45 UNICORN\n
T13:45\n
[download]

... which I can only presume is what you expect the output to be. Enjoy.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.