Sounds like a bug to me as I get an error message here

sub subname {
  local *_=\$_[0];
  s///;
}
subname("String");

Modification of a read-only value attempted at - line 3.
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This error should occurs because you're 'aliasing' $_ to $_[0], which essentially equates to both pointing to the same scalar value e.g

sub {
  print "\$_[0]: ",\$_[0], $/;
  local *_= \$_[0];
  print "\$_:    ",*main::_{SCALAR}, $/;
  s///;
}->("foo");

__output__

$_[0]: SCALAR(0x8107ec8)
$_:    SCALAR(0x8107ec8)
Modification of a read-only value attempted at - line 5.
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HTH

_________
broquaint