Check the result of prepare and execute. I bet you're getting a syntax error, and the fetch is giving you that error. More specifically, you're not escaping $X_Y and $check, and you're not quoting $check. I prefer replaceable parameters instead of building a SQL string dynamically where possible. If you use replaceable parameters, you don't have to quote or escape, and you're safe from SQL injection attacks.

$sql= "SELECT C.mast1, C.mast2 FROM MASTER WHERE C.mast1=? AND C.mast2
+=?";
$sth = $dbh->prepare($sql)
or die(...);
$sth->execute($check, $X_Y)
or die(...);
...
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