But, by limiting your customization technique to callbacks, all shared state must be stored in the lexical scope of the caller.

($foo, $bar)  = incrementByOneAndTwo(42);
($baz, $quux) = incrementByOneAndTwo(0);

$\="\n";
print $foo->();
print $bar->();
print $quux->();
print $foo->();
print $baz->();
print $bar->();
print $quux->();

sub incrementByOneAndTwo
{
    my $shared_state = shift;

    return (sub{$shared_state+=1}, sub{$shared_state+=2});
}
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