With warnings and use diagnostics I get this explanation:

Variable "$sep" is not available at (eval 9) line 1 (#1)
    (W closure) During compilation, an inner named subroutine or eval is
    attempting to capture an outer lexical that is not currently available.
    This can happen for one of two reasons. First, the outer lexical may be
    declared in an outer anonymous subroutine that has not yet been created.
    (Remember that named subs are created at compile time, while anonymous
    subs are created at run-time.) For example,
    
        sub { my $a; sub f { $a } }
    
    At the time that f is created, it can't capture the current value of $a,
    since the anonymous subroutine hasn't been created yet. Conversely,
    the following won't give a warning since the anonymous subroutine has by
    now been created and is live:
    
        sub { my $a; eval 'sub f { $a }' }->();
    
    The second situation is caused by an eval accessing a variable that has
    gone out of scope, for example,
    
        sub f {
        my $a;
        sub { eval '$a' }
        }
        f()->();
    
    Here, when the '$a' in the eval is being compiled, f() is not currently being
    executed, so its $a is not available for capture.
    
Use of uninitialized value $sep in concatenation (.) or string at (eval 9) line
        1 (#2)
    (W uninitialized) An undefined value was used as if it were already
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