The expression is evaluated before the loop starts in order to build the list of items over which to iterate,
and the expression returns either the result of keys or a single scalar depending on whether conditional's condition is true,
so it either loops for every key or just once.

No, because the ?: delivers either a list consisting of the single value or all the keys to the 'foreach' loop statement . So if that is only one value, the loop executes only for that single value.

Excellent, thanks.

I'll read up on this notation.




Time flies like an arrow. Fruit flies like a banana.

Some maintainability may be gained at the cost of some concision by separating out loop list generation:

my @keys_to_process = $x eq 'key_1'       ? keys %my_hash
                    : exists $my_hash{$x} ? ($x)
                    : ()  # default - nothing to do
                    ;

foreach $key (@keys_to_process) {
    do_something_with($key);
    }
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Further, because the above will loop over all keys if $x eq 'key_1' is true even if 'key_1' does not exist in the hash (which may or may not be what you want: Your Logic May Vary), perhaps re-order list generation:

my @keys_to_process = ! exists $my_hash{$x} ? ()
                    : $x eq 'key_1'         ? keys %my_hash
                    :                         ($x)
                    ;
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You can chain these suckers... so to ensure you do nothing if $x doesn't appear in %my_hash:

foreach $key ( ($x eq 'key_1') ?  keys %my_hash : exists $my_hash($x) 
+? $x : () ) {
  do a bunch of stuff
}
[download]

noting that if $x does not exist, what you want is an empty list.