use IPC::Open3 qw( open3 );

my $log = "..." . get_unique_str() . ".txt";

open(local *TO_CHLD, '<', 'nul') or die;
open(local *FR_CHLD, '>', $log)  or die;
waitpid(open3('<&TO_CHLD', '>&FR_CHLD', undef, @cmd), 0);