I used strict and warning.

And the call of the build_sth('set_to_flag') in another function which is called in the main per script:

(continue with previous code)
sub somecall{
build_sth('set_to_flag')
}

somecall();

[download]
I have checked the previous version where I got trapped. I get the code fail when I move the hash outside of function: 
my %Op = (
        set_to_flag => \&write_flag,
        set_tree => \&write_sets,
        default => \&do_nothing
    );
sub build_sth {
    my ($op_code) = @_;
    
    my $op_exec = $Op{$op_code} ;
    if( defined $op_exec ){
        $op_exec->();
    }else{
        print "Oops\n";
    }

}

[download]
why the code fail in this case?

In reply to Re^2: Function reference undef when extract from hash? by Anonymous Monk
in thread Function reference undef when extract from hash? by Anonymous Monk

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