use strict;
use warnings;

my $str = "aaa authentication login\r\n ssid Rich\r\n authentication open\r\n authentication shared \r\n";
my $authentication = '';  
my @candidates = split /[\r\n] +/, $str;

/^authentication\s+(\w*)(?{$authentication.= $1 . ' '})/ for @candidates;

print $authentication;

##</code><code>##

for (@candidates) {
    $authentication.= $1 . ' ' if /^authentication\s+(\w*)/;
}

##</code><code>##

open shared