The point is that all your parameters, including %visited get flattened and passed as a single list:

($x, $y, key1,val1,key2,val2,...,$word)

Inside dfs(), Perl has no way to tell that the last value ($word) is not an element of the list that must be assigned to %visited, so it attempts to assign that too. Hence the odd number of elements.

$word will never be assigned a value.

Swap the order of %visited and $word inside dfs() and when you call dfs() and your code will (probably) work!