#!/usr/bin/perl 

use strict;
use warnings;
#use diagnostics; # optional

use Getopt::Std; ...

##</code><code>##

$ perl -Mstrict -Mwarnings -E '
my @in_strings;
while (<>) {
    push @in_strings, $_;
}
my $count = grep /authenticated/ => @in_strings;
say "count = $count";
'
qwerty
asdfgh
zxcvbn
authenticated
unauthenticated
not authenticated
count = 3