#!/usr/bin/perl -w
open (INPUT, "target.txt")   or die "can't read from target:$!\n";
open (OUTPUT1, ">file1.txt") or die "can't write to file1.txt:$!\n";
open (OUTPUT2, ">file2.txt") or die "can't write to file2.txt:$!\n";

#
#
my $firsthalf = 1;

while (<INPUT>) {
    if ($_ =~ /^DETS01/) {
        $firsthalf = 0;
    }

    if ($firsthalf) {
        print OUTPUT1 $_;
    }
    else {
        print OUTPUT2 $_;
    }
}
close (INPUT);
close (OUTPUT1);
close (OUTPUT2);