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";
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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 ;-)