use Modern::Perl;
use IO::All;

my $first_file  = io('./first.txt')->chomp  or die $!;
my $second_file = io('./second.txt')->chomp or die $!;

my $index = 0;
while ( my $first_line = $first_file->getline ) {
    my $second_line = $second_file->getline;
    if ( $first_line ne $second_line ) {
        say "Files differ at line $index";
        say "First:  $first_line";
        say "Second: $second_line";
        last;
    }
    $index++;
}