I used $X, $Y, and $Z just to indicate they can vary and that it doesn't matter where they come from. You didn't specify an actual source for the patterns. If you assign regexen and substitution strings to those variables, the code will work as written -- give or take an /e in the substitution.

The $line{$.} = undef construction just creates a key in the %line hash without associating a value with it. Nothing of the actual file but line numbers is being stored.