$ perl -E 'sub foo { $_[0]= 5 } foo(4)'
Modification of a read-only value attempted at -e line 1.

##</code><code>##

use Scalar::Util "readonly";
sub foo {
  printf "%s\n", readonly($_[0])? "readonly" : "writable";
  $_[0]= 5 if !readonly($_[0])
}
foo(4);
foo(my $x= 4);
print "$x\n";