This was my attempt

my $string = <<EOF;
xxxYYxxxYYxxx
     YY   YY
     4Y   4X
     4Y   4Y
     YY   YY
EOF

    print "1: $string";

    $string =~ s/ (?: (?<=x) | \G ) (Y) (?= Y* x) /x/gx;

    print "2: $string";

    $string =~ s/ (?<= ^ (.*) Y .* $ ^ \1 )
                  (4)
                  (?= .* $ ^ \1 Y .* $ )
            /Y/mgx;

    print "3: $string";
[download]

The first part to match the horizontal parts works fine. The second part to match the verticals doens't work though for two reasong

1. It is going to match the same string horizontally in each case up to the vertical replacement (which wasn't specified)
2. Perl gave this message "variable length lookbehind not implemented" when I tried it ;-)