use strict;
use warnings;

open my $fh, '<', 'filename.dat';
binmode($fh);

my $len = -s $fh;
my $split_length = $length / 5; #would split 10gb into 2gb chunks

my $split_fh = $fh . 'split'; #creates 'filename.split'
my $num      = '1';

for ( 1 .. 5 ) {
    read $fh, $buf, $split_length;

    open my $out_file, '>', $split_fh . $num; #vreates '$filename.split000, 001, 002 ect
    binmode($out_file);

    print $out_file, $buf;
    close($out_file);

    $num++;
}

close($fh);