Each of the sails is created by prepending a "\b" to the spinner character:

my @sails = map { "\b$_" } qw{- \ | /};
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"\b" is a backspace character; so, when each sail is printed, it deletes the previous one. In the initial state, "Running:" is followed by two spaces (print q{Running:  };); the second space is overwritten by the first sail.

"\b", and other special characters, are listed in perlop: Quote and Quote-like Operators.