open my $fh, '<', $file) or die "Can't read '$file': $!\n";

while(<$fh>) {
    write_record($fh,$headerfile) if /$start_tag/;
    $headerfile++;
}

sub write_record {
    my ($fh, $outfile) = @_;
    my %out;
    $. = 0;
    while(<$fh>) {
        my ($key, $value) = split;
        $out{$key} = $value;

        if ($. == 17) {
            open my $header, '>', $outfile
                or die "Can't write '$outfile': $!\n";
            printf $header $headerformat, map { $_,$out{$_} } keys %out;
            close $header;

            write_detail($fh,$detailsection);

            $detailsection++;
            return;
        } 
    }
}

sub write_detail {
    my ($fh, $detailfile) = @_;
    open my $detail, '>', $detailfile
        or die "Can't write to '$detailfile': $!\n";
    while(<$fh>) {
        last if /$end_constant/;
        print $detail;
    } 
    close $detail;
}