It really depends on what the OP really needs, but there is a danger of $flag being reset while it is reading the next "few" lines, if the regex matches again. Perhaps something like this:

use strict;
my $few = shift || 3;
while (<DATA>) { 
  if (/Update certificate/) {
    print $_;
    print scalar <DATA> for 1..$few;
  }
}
[download]

Or possibly for 1..$few-1;, depending on whether $few includes the matching line or not.