system does not send keys to other programs, so your approach cannot work.

I recommend using sfdisk instead of fdisk, because sfdisk takes all parameters on the command line.

If you really, really want to automate entering keys into fdisk, take a look at Expect, or treat fdisk as a pipe:

my $cmd = "/usr/sbin/fdisk -c $device";
open my $fdisk, "| $cmd"
    or die "Couldn't spawn [$cmd]: $!";
for my $command ("n", "p", "1", "1", "+G", "t", "83", "w") {
    print $fdisk "$command\n";
};
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