#!/usr/bin/perl -w

use strict;

my $in_file = 'in.dat';
my $out_file = 'out.dat';
my $spec_text = qr/special text here/;

while (1) {
    open(my $fh_in, '<', $in_file)
        or die("open failed: $!");
    my @lines = <$fh_in>;
    close($fh_in);

    if ($lines[$#lines] =~ $spec_text) {
        open(my $fh_out, '>', $out_file)
            or die("open failed: $!");
        print $fh_out @lines[($#lines - 10) .. $#lines];
        close($fh_out);
    }

    sleep 60 * 30; # 30 minutes
}