perldoc -f exists

exists EXPR
             Given an expression that specifies a hash element or
             array element, returns true if the specified element
             in the hash or array has ever been initialized, even
             if the corresponding value is undefined.  The
             element is not autovivified if it doesn't exist.

                 print "Exists\n"    if exists $hash{$key};
                 print "Defined\n"   if defined $hash{$key};
                 print "True\n"      if $hash{$key};

                 print "Exists\n"    if exists $array[$index];
                 print "Defined\n"   if defined $array[$index];
                 print "True\n"      if $array[$index];

             A hash or array element can be true only if it's
             defined, and defined if it exists, but the reverse
             doesn't necessarily hold true.

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