package Foo;

# your posted code here

package main;

my $foo = Foo->new;
$foo->{_error_excerpt} = "hi";
say $foo->error_excerpt(undef); # No Error Excerpt.
say $foo->error_excerpt(1);     # hi
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Is that not what you expect to see? Perhaps you should put a print inside error_excerpt and make sure that $error_excerpt really has the value you think it has.