This reeks of being a homework assignment, so my assistance will be limited to two items:

1: To generate a 2 digit random number, you could use something like:

$x = int(rand(100));
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You'll need the number, so hang onto it.

2: Read the Perl documentation. You could do something like this:

$y =~ s!^\d{2}!$x!;
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where $x is your random value and $y the line you're changing.

The actual i/o and program logic will be left as an exercise.