You're right. That looks horrible. Maybe you should grab your value into a sensibly named lexical first, and reassign later so the whole operation looks clearer:

my $entity = 
    $self->{blablabla}{more_bla}{and_more}[$_];

$entity = 'a' . $entity;

$self->{blablabla}{more_bla}{and_more}[$_] =
    $entity;
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This is marginally less efficient, particularly if the string in question is really big. But it deobfuscates what's going on. It would be really easy to miss the concatenation if you're reading through your code. But if you unpack your value into some sensibly named lexical before performing operations on it, the resulting code will be much easier to read.