Worked fine for me simply by adding the missing "\n" I mentioned.

I had problem detecting the prompt because it's not sent until the line is ended (which doesn't happen until someone types since the output appears to be line buffered). I solved that by forcing the prompt to show up twice.

use strict;
use warnings;

use IO::Handle qw( );
use IPC::Open2 qw( open2 );


sub wait_for_prompt {
    my ($to_chld, $fr_chld) = @_;

    # Force two prompts to appear.
    print $to_chld "\n";

    my $count;
    while (<$fr_chld>) {
        print "$.: $_";
        last if /^DISKPART> $/ && ++$count == 2;
    }
}


{
    my $pid = open2(my $fr_chld, my $to_chld, 'diskpart');

    #print $to_chld "\n";
    #wait_for_prompt($to_chld, $fr_chld);

    print $to_chld "list disk\n";
    wait_for_prompt($to_chld, $fr_chld);

    print $to_chld "exit\n";
    wait_for_prompt($to_chld, $fr_chld);

    close $to_chld;
    waitpid($pid, 0);
}
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