On a related note: Any ideas why this:

    while( substr( $$str, $y * $WIDTH, $WIDTH ) =~ m[((.)\2*)]mg ) {
    ...
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Works (runs to completion, produces the desired results) on 5.10.1, but silently loops forever in 5.22.0?

Looks like a bug fix. expr =~ /.../g attaches position magic to the LHS after each match, to record the current pos() for that expression. If expr is a var or similar this works well; if expr returns something new each time, the old pos magic gets lost.

This similarly loops forever:

sub f { "abc" }

1 while f() =~ /./g;
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Dave.