my $outfilename = "outfilename";

open INFILE, "$input_file" 
    or die "Can't open $input_file: $!\n";

while (<INFILE>) {
    if ( /section-end-marker/ ) {
       close OUTFILE;
       next;
    }
    if ( /section-start-marker/ ) {
        # generate the new $outfilename however
        open OUTFILE, "> $outfilename" or die"$!\n";
        next;
   }
    print OUTFILE;
}