The Perl code that converts strings to integers specifically checks for "0 but true". "0 but True" will generate a warning in a numeric context, for example:

  % perl -we "print 0+'0 but True'"
  Argument "0 but True" isn't numeric in addition (+) at -e line 1.
  0
  % perl -we "print 0+'0 but true'"
  0
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        - tye (but my friends call me "Tye")