If you need to play with long strings char-by-char, spliting and joining them can consume large amounts of memory, and making every reference to each char of the form substr( $s, $p, 1 ) = 'X' if substr( $s, $p, 1 ) ne ' '; can be a PITA.

You can avoid both using lvalue substr refs.

my $s= 'the quick brown fox jumps over the lazy dog';

$$_ ne ' ' and $$_ = 'X' 
    for map{ \substr( $s,$_, 1 ) } 0 .. length $s;

print $s;
XXX XXXXX XXXXX XXX XXXXX XXXX XXX XXXX XXXX
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I'm still hoping that LW will see fit to include $s[ $p ] syntax for accessing the chars of a string in P6 once that syntax is no longer valid for elements of an array -- or at least that it will be possible to create a module that gives this syntax efficiently.