A solution with s/// seems inefficient. And the loop is probably unavoidable: you'd need either variable lookbehind, or multiple matches on same text. Anyway:

my $str = "abcdefghiaabccdjklm";
print "Input : $str\n";
1 while $str =~ s/(.)(.*?(?=\g1))((?=.+?\g1)|.)/$2/;
print "Output: $str\n";
[download]