use strict; 
use warnings; 

#set up a pretend subref inside some other stuff
my $bar = {foo => [ sub {print "fishcake"}]};

{
   no warnings "uninitialized"; # we're going to be a bit naughty in t
+his block
   ($bar->{potato}->[0] or sub {return undef})->() 
}


# swap this for
# ($bar->{foo}->[0] or sub {return undef})->()
# and see what happens
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if you use that then if I were you I'd comment it liberally, and possibly surround it with bio-hazard marker tape.