grep returns aliases, but assignment (to @m) makes copies. You can not modify the grep results directly though, so you need to do something like take a reference, pass it to a (naughty) sub, or make a modifiable alias in order to modify the alias, and not just a copy thereof:

@l = ( "a", "b", "c" );
$_ = "z" for grep /a/, @l;
print @l;
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Prints "zbc".