Once you've read a line from <IN>, it is "used up", and the next call to <IN> will return the line after it.

So when you do

while ($line = <IN>)
{$nextline=<IN>;
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You're reading two lines, and so overall $nextline will only contain every second line in the file (all even-numbered lines if you start counting from 1).

So if all the matches are on odd-numbered lines, you won't see anything ini the output.

Try something like this instead:

my $previous = '';
while ($line = <IN>) {
     if ($line =~ /mamma/ && $line =~ /mia/) {
          print OUT $previous;
          print OUT $line;
     } else {
          print ".";
     }
} continue {
    $previous = $line;
}
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That way $line sees all the lines in the input file, and you still have the previous line available inside the loop.