use IO::File;
....
$IPcheck = 0;
{
  my $file = new IO::File "$temp", "r";
  while (<$file>)
  {
    chomp;  # operates on the $_ variable directly
    if ($_ eq $ipaddress)
    {
      $IPcheck = 1;
      last;
    }
  }
}
# note that you do not have to close the $file handle, falling
# out of the scope and perl will automatically close the file.