why the code fail in this case?

The only possibly reason is if build_sth('set_to_flag') is called before my %Op = ( ... ); is executed.

Otherwise, it doesn't fail.

sub write_flag { print "write_flag\n"; }
sub set_tree   { print "set_tree\n"; }
sub do_nothing { print "do_nothing\n"; }

... code you provided ...

build_sth('set_to_flag');  # Prints write_flag
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