Variable length lookbehind not implemented in regex; marked by <-- HERE in
        m/=~\s*s(.)(?<!\1) <-- HERE / at script.pl line 8 (#1)
    (F) Lookbehind is allowed only for subexpressions whose length is fixed and
    known at compile time. The <-- HERE shows in the regular expression about
    where the problem was discovered. See perlre.

Uncaught exception from user code:
        Variable length lookbehind not implemented in regex; marked by <-- HERE
in m/=~\s*s(.)(?<!\1) <-- HERE / at script.pl line 8.