opendir DIR, "DIR_A";

while(defined(my $item = readdir DIR)){
    open FILE,"DIR_A/$item" or warn "$!", next;
    open COPY,"COPIES/$item" or warn "$!", next;
    while (read FILE,my $buffer,4096){
        print COPY $buffer;
    }
    close FILE;
    close COPY;
    exec("mv","DIR_A/$item","DIR_B/$item"); # performes an mv command like on the command line
}

closedir DIR;