c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my $s = 'abXXcXXXXdefXXXXgXXhiXXXXXXXjkXX';
 print qq{$s};
 ;;
 my @off_len;
 while ($s =~ m{ X+ }xmsg) {
   push @off_len, [ $-[0], $+[0]-$-[0] ];
   }
 dd \@off_len;
 ;;
 for my $ar_ol (reverse @off_len) {
   my ($off, $len) = @$ar_ol;
   substr $s, $off, $len, 'YYYYY';
   }
 print qq{$s};
"
abXXcXXXXdefXXXXgXXhiXXXXXXXjkXX
[[2, 2], [5, 4], [12, 4], [17, 2], [21, 7], [30, 2]]

abYYYYYcYYYYYdefYYYYYgYYYYYhiYYYYYjkYYYYY