You have to count all the lines of the file, you can use $. for that, heres an example withoout reading all file in an array:

#!/usr/bin/perl -w
use strict;
open(FILE,"file.txt") or die "Cant open file.txt: $!";
1 while <FILE>;
my $line=$.-30;
$.=0;
open(FILE,"file.txt") or die "Cant open file.txt: $!";
open(FILE2,">file2.txt") or die "Cant create file2.txt: $!";
while(<FILE>)
{
    print FILE2 $_;
    insert(*FILE2) if $.==$line;
}
rename("file2.txt","file.txt") or die "Cant rename file: $!";

sub insert
{
    my $fh=shift;
    print "Inserting at line $line\n";
    print $fh "$_\n" for 0 .. 15;
}
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$anarion=\$anarion;

s==q^QBY_^=,$_^=$[x7,print