#!/usr/bin/perl
use strict;
use warnings;
use autodie;

# Open file in read/write mode
open my $FH, '+<', 'tmp.txt';

# Skip the first line
my $t = <$FH>;

# Remember the starting location of the second line
my $pos = tell $FH;

# Read the second line
$t = <$FH>;

# Rewind back to the start of the second line and obliterate it
seek $FH, $pos, 0;
print $FH "*" x (length($t)-length($/));