Unless you want to modify default variable $_ & save the number of replacements, there is typo in your s/// application. In that case, op should be (to save the result of all of replacements in another string) ...

( my $fixed = $input ) =~ s/ search / replace /x;
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For fun, try the proposed s/(\s)/\\$whitespace{$1}/g solution on string x y (a plain space in between).