Do you mean this?

$ perl -E 'sub foo { $_[0]= 5 } foo(4)'
Modification of a read-only value attempted at -e line 1.
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I would call '4' a "constant", though it appears perl calls it a "read-only value". You can find out whether one of your parameters is readonly like this:

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";
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Maybe tinyperl's errors are different. If you had an error somewhere like "Can't use a constant as an LVALUE" I can see where that might have been confusing, but LVALUE is referring to "a thing which can be assigned-to", and that error would be saying basically that you can't use a read-only or constant on the left side of an equal sign.