Most of the above thread is, err, basically wrong! The two Concise outputs show that the only two ops executed in each case are the GV (to get the glob containing the global hash variable) followed by a MULTIDEREF op. The ex-helem and ex-exists are ops that were optimised away and replaced with the MULTIDEREF op (on perl 5.22.0 and later, anyway) - they are no longer on the execution path.

The difference in performance between exists and a plain hash lookup is negligible, although exists should be slightly faster. The slowdown being seen is due to assigning a boolean return value to a scalar. Perl's boolean values (as returned by exists etc) are multi-valued scalars containing an int value of 0/1, a floating value of 0.0/1.0 and a string value of ""/"1". If this value is assigned to a scalar, all three parts need to be copied, including the string. Boolean values are optimised for use in conditionals. If you replace the assignment with something like $xx_global = exists $hash{$key2} ? 1 : 2 you'll see exists becomes faster than a hash lookup.

Dave.