my $foo = "x.....xxx.x.....x......xxx...xx...x...xxx";

my @array =
$foo =~ 
/(                              # capture in $1
[x]                             # match an opening x
        (?:[^x]*?[x][^x]*?)     # match an x surrounded by some non-x
        {2}                     # match n-2 times
[x]                             # match a closing x
)/gx;
#(?{push @array, $1})            # keep the match
#$                               # match the end of the string to force back-tracking

print "$_\n" foreach (sort {length $a <=> length $b} @array);

__END__
x.....xxx
x...xx...x
x.....x......xx


##</code><code>##

my $foo = "x.....xxx.x.....x......xxx...xx...x...xxx";

my @array;
$foo =~ 
/(                              # capture in $1
[x]                             # match an opening x
        (?:[^x]*?[x][^x]*?)     # match an x surrounded by some non-x
        {2}                     # match n-2 times
[x]                             # match a closing x
)
(?{push @array, $1})            # keep the match
$                               # match the end of the string to force back-tracking
/x;
print "$_\n" foreach (sort {length $a <=> length $b} @array);

__END__
xxx.x
xxx...x
xx...xx
xx...xx
xx...xx
xx...xx
x...xxx
x.....xxx
xx.x.....x
xx.x.....x
x......xxx
x...xx...x
xx...x...x
xx...x...x
xx...x...x
xx...x...x
x...x...xx
x...x...xx
x...x...xx
x...x...xx
x.....x......xx
x.....x......xx
x.....x......xx
x.....x......xx
x.....x......xx
x.....x......xx
x.....x......xx
x.x.....x......x
x.x.....x......x
x.x.....x......x
x.x.....x......x
x.x.....x......x
x.x.....x......x