"Glob" is another name for "symbol table entry". Symbol table entries have multiple fields, one for each data type (scalar, hash, array, etc).

$abc = 'xyz';
print(${*{abc}{SCALAR}}, "\n");  # xyz
[download]

When you modify a package variable, you modify the data in the appropriate field of the appropriate symbol table entry.

$glob = *def = *abc;
$abc = 'xyz';
print($abc               , "\n");  # xyz
print($def               , "\n");  # xyz
print(${$glob           }, "\n");  # xyz
print(${*{abc  }{SCALAR}}, "\n");  # xyz
print(${*{def  }{SCALAR}}, "\n");  # xyz
print(${*{$glob}{SCALAR}}, "\n");  # xyz
[download]

Same goes when you localize a package variable. You're localizing the SCALAR field of the symbol table entry. It doesn't matter how you get to the symbol table entry (be it $_ or $alias0, for example), because the value is not associated with the symbol ($_), but with the symbol table entry (${*_{SCALAR}}).

$glob = *def = *abc;
$abc = 'xyz';
print($abc               , "\n");  # xyz
print($def               , "\n");  # xyz
print(${$glob           }, "\n");  # xyz
print(${*{abc  }{SCALAR}}, "\n");  # xyz
print(${*{def  }{SCALAR}}, "\n");  # xyz
print(${*{$glob}{SCALAR}}, "\n");  # xyz
local $def = 'uvw';
print($abc               , "\n");  # uvw
print($def               , "\n");  # uvw
print(${$glob           }, "\n");  # uvw
print(${*{abc  }{SCALAR}}, "\n");  # uvw
print(${*{def  }{SCALAR}}, "\n");  # uvw
print(${*{$glob}{SCALAR}}, "\n");  # uvw
[download]

In short, *abc = *def; makes the symbols abc and def the same. Only changes to the globs can undo this. local $abc doesn't affect the glob *abc (or *def), only the scalar component of it.