Best thing would be is to open the file in the append mode ,, Not a fan of the previous answer , for one reason that it will not work :

!/usr/bin/perl 
use strict;

open (FILE, "file") || die "File does not exist\n";
my @file = <FILE>;
my $file = @file;
close (FILE);

open (FILE, ">file") || die "File does not exist\n";
for (my $n = 0; $n <= $file; $n++) {
  $_ = $file[$n];
  print FILE $_;
}

print "add whatever"; -->  should be print FILE "add whatever"; 
print "here\n"; --> should be print FILE "here\n";
print "to end of txt.\n"; --> should be print FILE "to end of txt.\n";

close (FILE);
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Originally posted as a Categorized Answer.