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$ demo() {
   perl -e'
...

$ PERL_HASH_SEED=0x0123456789ABCDEF demo
lfojtgpzekwriyvchmnsuadbqx

BEGIN {
   return if exists( $ENV{ PERL_HASH_SEED } );
...
   exec( $^X, "--", $0, @ARGV )
      or die( "exec: $!" );
}