All squares that match a wildcard sequence
Mathematica, 44 bytes
Print@@@IntegerDigits[Range@1*^3^2]~Cases~#&amp;

The input is a list of digits with a _ (no quotes) as a wildcard. e.g. {4, _, _, 6}
Explanation
Range@1*^3

Generate list {1, 2, 3, ... , 1000}
... ^2

Square it. (list of all squares from 1 to 1,000,000)
IntegerDigits[ ... ]

Split each square into a list of digits.
... ~Cases~#

Find the ones that match the pattern specified by the input.
Print@@@ ...

Print them.