#!/usr/bin/perl

use strict;
use warnings;

my $out = join '', <DATA>; # fake data :)

my @essids = map /SSID: (.*)/, split /^BSS/m, $out;

use Data::Dumper;
print Dumper \@essids;

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