... I still get the same error as described initially ...

From the OP: '... I'm getting a "Use of uninitialized value $_ in pattern match (m//)" error.'
If you're really still getting this exact "error" (a warning, actually), then you still have a
    /MV(\d{4})/;
statement lurking somewhere in your code that is still trying to implicitly match against the  $_ variable which you have still not initialized. What line number is associated with the warning message? Look at (or near) that line for the offending statement.