even though the container is immutable

It isn't. It looks like splice doesn't care about SvREADONLY at all (but unshift does). You can extend such a list an array via splice:

@a = qw[a simple list];
warn join ' ', @a;               # "a simple list"
Internals::SvREADONLY(@a,    1); # Make the list RO (doesn't matter he
+re, actually)
Internals::SvREADONLY($a[1], 1); # Make our target element RO (double 
+sure!)
warn '@a is ' . (Internals::SvREADONLY(@a) ? '' : 'not ') . 'read-only
+';
splice(@a, 1, 1, qw(not quite readonly));
warn join ' ', @a;               # "a not quite readonly list"
warn '@a is ' . (Internals::SvREADONLY(@a) ? '' : 'not ') . 'read-only
+';
unshift @a, qw*this is*;
__END__
a simple list at 1167665-reply.pl line 2.
@a is read-only at 1167665-reply.pl line 5.
a not quite readonly list at 1167665-reply.pl line 7.
@a is read-only at 1167665-reply.pl line 8.
Modification of a read-only value attempted at 1167665-reply.pl line 9
+.
[download]

This is a bug (incomplete implementation, maybe also a design issue). It looks like Internals::SvREADONLY has been tacked onto, not "bolted through" the perl source