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use strict;
use warnings;
...
my @segments = map {[$_, (scalar tr/STY//)]} split /(?<=[KR])/, $str;

printf "%45s: %d\n", @$_ for @segments;

                                 GVGFTVILISFK: 2
                  YVGFFYNVIIAWALHYFFSSFTMDLPR: 6
   WIHCNNTWNSPNCSDAHASNSSDGLGLNDTFGTTPAAEYFER: 10