use Carp qw/croak/;
BEGIN {
    my $logger;

    sub callback {
        my $coderef = shift;
        croak "$coderef is not a code reference, cannot use as a callback.\n"
                unless ref($coderef) eq 'CODE';
        $logger = $coderef;
    }

    callback( sub { print STDERR "Executing $_[0]!\n" } );

    sub logCall {
        &$logger( @_ );
    }
}