Well, this you'll have to do before you call it(and you might want it in a BEGIN block if it's for the whole script):

undef testsub();
sub testsub { eval 'whatever action you want to take' }
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If this is for a small area of the program, wrap it in enclosing blocks, i.e., {}. Also, you may wish to put a 'local' in front of sub testsub.