Use a look-ahead to make sure you COULD match the repeated letter, but not actually advance in the string:

$str =~ s/(.)(?=\1)/$1x/g;
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_____________________________________________________
Jeff japhy Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;