It's probably best to work with smaller chunks than 200 MB. There's no reason why you can't convert the files using something like:

use strict;
use warnings;

my ($fsize, $csize, $file, $fcount, $read, $in, $out, $chunk);

$fsize = 1024 * 1024 * 200; # Size of file you want
$csize = 1024 * 1024;       # Size of chunks
$/ = "\n";                  # String to stop at
$file = "test.dat";
$fcount = 1;                # Number for file names

$read = 0;
$out = nextfile($file, $fcount);
open($in, $file);
while (read($in, $chunk, $csize) == $csize) {
    print $out $chunk;
    $read += $csize;
    if ($read >= $fsize) {
        print $out $_ = <$in>;
        $read = 0;
        close($out);
        $out = nextfile($file, $fcount);
    }
}
close($in);
close($out);

sub nextfile {
    my ($name, $handle) = $_[0];
    $name =~ s/\./$_[1]./;
    $_[1]++;
    open($handle, ">$name");
    return $handle;
}
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Note that this version includes the stop string in the previous file. If you want it included in the next file:

    if ($read >= $fsize) {
        chomp($_ = <$in>);
        print $out $_;
        $read = 0;
        close($out);
        $out = nextfile($file, $fcount);
        print $out $/;
    }
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