Depending on what you are doing, you might find it cleaner to extract all same-letter sequences, then filtering out those of the incorrect length.

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

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

perl -e'
   use v5.14;

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