if ( grep length( $_ ) == 2, /([A-Za-z]\1+)/g ) {
   say "match";
} else {
   say "no match";
}

##</code><code>##

/([A-Za-z])\1(?:\1+(*SKIP)(*FAIL))?/

##</code><code>##

perl -e'
   use v5.14;

   for ( "aa", "bbb", "cccc", "deefffgggghhiiiii" ) {
      say s/([A-Za-z])\1(?:\1+(*SKIP)(*FAIL))?/[$&]/gr;
   }
'

##</code><code>##

[aa]
bbb
cccc
d[ee]fffgggg[hh]iiiii