use strict; use warnings; use feature 'say';
my $input = do { local $/; <DATA> };

my @output = map { /(\S+)$/ } grep { /SSID/ } split /\n/, $input;

say for @output;

__DATA__
BSS <blah blah>
   SSID: <ssid>
   <blah blah>
BSS <blah blah>
   SSID: <ssid2>
   <blah blah>
BSS <blah blah>
   SSID: <ssid3>
   <blah blah>