1) The code you posted doesn't exhibit the problem, and 2) the complaint is different that what you say it is.

The problem is found in code of the following form:

package Foo;
sub sort($&) {
   my ($self, $cmp) = @_;
   ...
   my @sorted = sort $cmp @list;    <-- sort compiled in one package
   ...
}

package Bar;
Foo->sort({ $a cmp $b });           <-- Sub compiled in another
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sort sets $a and $b of the package in which the sort is present.

The simplest solution (while continuing to use $a and $b) is

package Foo;
sub sort($&) {
   my ($self, $cmp) = @_;
   ...
   my $caller = caller;
   my @sorted = eval("package $caller; sort \$cmp \@list");
   die $@ if $@;
   ...
}
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Try something like:

package Foo;

...

sub sort {
    my $self = shift;
    my $code = shift;
    sort { $code->($a, $b) } @{$self->{arrayref}};
}

my $foo = Foo->new;
$foo->sort(sub { lc($_[0]) cmp lc($_[1]) });
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I know it's not as pretty, but it should work.

How does Perl's built-in sort manage to be prettier than code you've written yourself? The answer's in the question!

Although it might work and this is unlikely to be the cause of your problem, I do not think that naming your own function or method with the name of a built-in function is very good practice. It might obscure any issue. An additional comment is why are you doing this:

my $code = \&{shift @_};
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my $code = shift;
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