How about something resembling:

#!/usr/bin/perl -w

open INPUT, "target.txt" || die "can't read from target:$!\n";
open OUTPUT1, ">file1.txt" || die "can't write to file1.txt:$!\n";
open OUTPUT2, ">file2.txt" || die "can't write to file2.txt:$!\n";
#
#
while (<INPUT>) {
    last if $_ =~ /^DETS01/;
    print OUTPUT1 "$_\n";
}
while (<INPUT>) {
    print OUTPUT2 "$_\n"
}

close INPUT;
close OUTPUT1;
close OUTPUT2;
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This doesn't put the line containing DETS01 in either file, so change it if it should.