When asking a question like this it's best to include the code you are trying to get working. Else we have little idea where your specific problem is and how to guide you.

Generally if the file isn't too big just feed it into an array. Then append the array with your new info and print it out.

use strict;
use warnings;

open(FH,'<','input') or die "$!\n";
my @array = <FH>;
close FH;
push(@array,"Your new data\n");
open(OUT,'>','output') or die "$!\n";
print OUT @array;
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Try reading: How (Not) To Ask A Question and I know what I mean. Why don't you?