I tested in 5.8.7, not 5.8.8, but I saw the same results. I tested a few other things about X::a, but I can't see why the debugger gets confused.

One clue as to what's going on might be in the perl -MO=Deparse output:

package X;
sub BEGIN {
    require strict;
    do {
        'strict'->import
    };
}
use Attribute::Handlers;
sub a {
    use strict 'refs';
    print "a\n";
}
use attributes ('X', sub {
    use strict 'refs';
    print "a\n";
}
, 'ATTR(SCALAR)');
sub b {
    use strict 'refs';
    print "b\n";
}
package main;
use strict 'refs';
X::a();
X::b();
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The use attributes after sub a is interesting. If it reassigned *X::a that could explain weird behaviour, but it doesn't. Interesting!

Adding a call to a routine that prints "caller" info instead of the print statements in a and b shows that the caller info is sane, and matches the expected line numbers.