Remove the -1:

sub shuffleWord {
    my( $l, $p ) = length( $_[0] );

    $p = int rand $l - $_ and
    substr( $_[ 0 ], $_     , 1 ) ^= 
    substr( $_[ 0 ], $_ + $p, 1 ) ^= 
    substr( $_[ 0 ], $_     , 1 ) ^= 
    substr( $_[ 0 ], $_ + $p, 1 ) for  0 .. $l;

    return $_[ 0 ];
}
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