I've got an edge case no one in their right mind would stumble upon. This time an oldie from the days when Readonly::XS was needed but still matters today for Readonly (edit: which I currently maintain), Const::Fast (edit: which is the result of Readonly's inherently broken modus operandi), etc.; modules that wrap perl's own internal means of marking variables as read-only. Consider this: 
    my @a = qw[a simple list];
    Internals::SvREADONLY(@a,    1);
    Internals::SvREADONLY($a[1], 1); # Make our target element RO (dou
+ble sure!)
    warn '$a[1] is ' . (Internals::SvREADONLY($a[1]) ? '' : 'not ') . 
+'read-only';
    warn join ' ', @a;               
    eval { $a[1] = 'changed' };      # Modification of a read-only val
+ue attempted
    warn join ' ', @a;               
    splice(@a, 1, 1, 'spliced');
    warn join ' ', @a;               
    warn '$a[1] is ' . (Internals::SvREADONLY($a[1]) ? '' : 'not ') . 
+'read-only';

[download]
...the output of the above reads... 
    $a[1] is read-only at debug.pl line 3.
    a simple list at debug.pl line 4.
    a simple list at debug.pl line 6.
    a spliced list at debug.pl line 8.
    $a[1] is not read-only at debug.pl line 9.

splice(...) can replace read-only elements where setting them with an operator throws an exception because the actual element is replaced internally rather than just the value. ...even though the container is immutable. splice(...) can also delete elements from a immutable array where delete() will fail: 

    my @b = qw[another simple list];
    Internals::SvREADONLY(@b, 1);# Matters here
    warn join ' ', @b;           # "another simple list"
    eval { delete $b[1] };       # throws Modification of...
    warn join ' ', @b;           # "another simple list"
    splice(@b, 1, 1);            # Zot!
    warn join ' ', @b;           # "another list"

[download]
It's been a long day but to me, this is unexpected behavior. It's certainly low priority (due to it being a feature of Internals::) but before I report it as a bug on RT, I'm seeking opinions.

Code tested on Strawberry Perl v5.20.1 (MSWin32-x64-multi-thread) but this originally came to me years ago.

In reply to Calling splice() on Immutable Arrays by SankoR

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