Well, for starters, you're using 5, 4, and *, rather than 10 and 20. Secondly, you're ignoring the warning message:

\1 better written as $1 at ./z.pl line 7.
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(You are using strict and warnings, right?) Third - what's with the ($a = $a )? Finally, you've inverted the problem. You're replacing everything except characters 6 through 9. You probably want something like:

  $a =~ s/^(\S{5})\S{4}/$1xxxx/;
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Or, potentially simpler:

  $a =~ s/(?<=^\S{5})\S{4}/xxxx/;
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But what you really want is still substr.