pdl> apropos bad   # PDL shells have online help
PDL::Bad        Module: PDL does process bad values
PDL::BadValues  Manual: Discussion of bad value support
badflag         getter/setter for the bad data flag
badinfo         information on the bad-value support
...many more...

pdl> help isbad
Module PDL::Bad
  isbad
      Signature: (a(); int [o]b())

    Returns a binary mask indicating which
    values of the input are bad values

    Returns a 1 if the value is bad, 0 otherwise.
    Similar to isfinite.

     $a = pdl(1,2,3);
     $a->badflag(1);
     set($a,1,$a->badvalue);
     $b = isbad($a);
     print $b, "\n";
     [0 1 0]

    This method works with input piddles that
    are bad. The output piddle will never contain
    bad values, but its bad value flag will be the
    same as the input piddle's flag.

pdl> $data = rint(10*random(10))

pdl> p $data
[5 9 8 3 5 6 7 7 6 10] 

pdl> $special = 7

pdl> p $data->setvaltobad($special)
[5 9 8 3 5 6 BAD BAD 6 10] 

pdl> p $data->setvaltobad($special)->list
5 9 8 3 5 6 BAD BAD 6 10 

pdl> @pdata = $data->setvaltobad($special)->list

pdl> p "@pdata"
5 9 8 3 5 6 BAD BAD 6 10 

pdl> foreach (@pdata) { $_ = undef if $_ eq 'BAD' }

pdl> p "@pdata"
Use of uninitialized value $pdata[6] ...
Use of uninitialized value $pdata[7] ...
5 9 8 3 5 6   6 10 


pdl> p which $data==$special  # calc indices of "special vals"
[6 7] 

pdl> @ordinary = $data->where($data != $special)

pdl> p "@ordinary"   # or output just ordinary values
[5 9 8 3 5 6 6 10]