while(<F>)
    {
    if(/IP Address/)
        {
        @split0= split(/ /, $_);
         @MYIP0=(@MYIP0,@split0);
        }
[download]

Is almost certainly not what you want. If you do match /IP Address/ and then split on a space and then append to the list you will get something like:

@MYIP0 = ( 'IP', 'Address', "192.168.1.1\r\n" )
[download]

at best. Then even if you do match your /\d*\.\d*\.\d*\.\d*/ you're still keeping the wrong value. You probably want something like:

while (<F> {
  if (/IP\sAddress\s+(\d+\.\d+\.\d+\.\d+)/) {
    push @MYIP, $1;
  }
  ...
}
$ff = $MYIP[0];
$fk = "$ff:255.255.255.0";

# even better, use 2 arg version of Net::Netmask->new2
$block = Net::Netmask->new2( $MYIP[0], '255.255.255.0');
...
[download]

The not perfect regex is OK since output from system commands rarely varies.