use strict;
use warnings;
use feature qw(say);

my $file    = q(big.txt);
my $size    = -s( $file);
my $pattern = qr((y+)(?:\n*)(y*));

open my $fh, '<', $file or die $!;
read( $fh, my $data, $size );
close $fh;

while( $data =~ m/$pattern/g ) {
  say qq($1$2);
}

__END__

# data like this: 
xyyyyyxxxx
xxxxxxxxxx
xxxxxxxxyy
yyyxxxxxxx
xxxxxxxxxx
xxxxxxyyyy
yxxxxxxxxx
xxxxxxxxxx
xxxxxyyyyy
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
# small version ;-)

Desktop\monks>1129652.pl
yyyyy
yyyyy
yyyyy
yyyyy