I guess because undef in an integer context will be taken as '0' (albeit with a warning if you have asked for warnings), whereas -1 is not a valid array index:

@b = (a .. z);
                                                                      
+          
$#b = undef;
                                                                      
+          
print "@b";
                                                                      
+          
$#b = -1;
                                                                      
+          
print "@b";
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/J\

That makes some sense. -1 is a valid index though, in normal use - it gives you the last element. But in the code you gave, the -1 indeed makes @b an empty array (I didn't know you could do that). Kind of like a quick way to truncate arrays.