If the log file is huge, it will be better to avoid the scalar variable altogether and to instead read the file in backwards, line by line. The File::ReadBackwards module on is designed for this purpose:

#! perl
use strict;
use warnings;
use File::ReadBackwards;

my $logfile = 'log.txt';
my $header  = 'start: real';
my $footer  = 'end: real';
my $in_data =  0;
my @lines;
my $bw      =  File::ReadBackwards->new($logfile)
    or die "Cannot open file '$logfile' for reading backwards: $!";

while (my $line = $bw->readline)
{
    chomp $line;

    if ($in_data)
    {
        unshift @lines, $line;
        last if $line eq $header;
    }
    elsif ($line eq $footer)
    {
        unshift @lines, $line;
        $in_data = 1;
    }
}

print join("\n", @lines), "\n";
[download]

No regex needed. :-)

Hope that helps,