Note that \r should generally be used for cr rather than \x0d.

The following uses split to break the string into candidate sub strings, then uses a regex with a code evaluation expression in it to extract the text you want. A more conventional alternative is given too.

use strict;
use warnings;

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

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

print $authentication;
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Alternate extract code:

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

open shared
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