sub write_flag{
	print "Success for flag.\n";
}
sub write_sets{print "Success for Set.\n"; }
sub do_nothing{print "Exit Here\n";}
my %Op = (
	set_to_flag => \&write_flag,
	set_tree => \&write_sets,
	default => \&do_nothing
);
sub build_sth{
	my ($op_code) = @_;
	print "\n\nCode: $op_code:\n";
	my $op_exec = $Op{$op_code} ;
	if(defined($op_exec)){
		print "it exist: $op_exec\n";
	}else{
		print "Oop\n";
	}

}

##</code><code>##

build_sth('set_to_flag');

##</code><code>##

Oop

##</code><code>##

Can't use string ("") as a subroutine ref while "strict refs" in use at ...