open (FILE,"external_file.txt");
open (TEMP,">external_file.txt.tmp");

my $i = 0;

while (<FILE>){ # magical - puts every line in $_, until the end of file is reached
	if ($i == 3){ # if $i is the 3rd line
		chomp $_; # remove the line break at the end of $_
		$_ .= "my new line goes here\n"; # add the string
	}
	print TEMP $_; # print the line you just read to the temporary file
} continue { # performed at the end of every loop iteration
	$i++;
}

close FILE;
close TEMP;

unlink ("external_file.txt"); # incase rename does not clobber, delete the original
rename ("external_file.txt.tmp","external_file.txt");