#!perl
# Untested
use 5.012;

my $partsize = 200 * 1024 * 1024;

my $file = shift or die 'no file';

open my $in, '<', $file or die "Can't open '$file' for reading: $!";

my $part = 1;
my $size = 0;
open my $out, '>', "$file.part$part" 
  or die "Can't open '$file.part$part' for writing: $!";
while (<$in>) {
    print $out $_;
    $size += length $_;
    if ( $size >= $partsize ) {
        close $out;
        $part++;
        open $out, '>', "$file.part$part" 
          or die "Can't open '$file.part$part' for writing: $!";
        $size = 0;
    }
}