use strict;
my $newcontent="My camel has been bitten by a rather large flea. Please call a vet.";
my $input="camel.cgi";
my $tempfile="/tmp/camel.$$";
open TARGET, "<$input" or die "Blerch: $!\n";
open TEMP, ">/$tempfile" or die "Hcerlb: $!\n";

while (<INPUT>) {

  # check for your cue: could be a running counter to
  # keep track of lines read, could be a regexp, could
  # be just about anything else. Let's assume the check
  # sets $ship_ahoy when you need to add lines.
  
  if $ship_ahoy {print TEMP $newcontent}
  print TEMP;
  }
close INPUT;
close TMP;
rename $tempfile $input or die "Hrlecb:$!\n";
print "Beware, this is untested code, and could definitely do with more robust error checking!\n";