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');
...