I'm starting with your original statements, and then setting the selected elements to '-1' here:

perldl> p $b = pdl (10..25)->reshape(4,4);
[
 [10 11 12 13]
 [14 15 16 17]
 [18 19 20 21]
 [22 23 24 25]
]
perldl> p $pos = whichND($b == 21); 
[
 [3 2]
]
perldl> $xfrm = pdl [-1,0],[0,-1],[1,0],[0,1];
perldl> p $around = $b->indexND($pos + $xfrm, 'truncate');
[20 17 0 25]
perldl> p $odd = $around->where( $around % 2 );
[17 25]
perldl> $odd .= -1;  # set original matrix through dataflow
perldl> p $b
[
 [10 11 12 13]
 [14 15 16 -1]   # odd elements around 21 are set to -1
 [18 19 20 21]
 [22 23 24 -1]
]
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