in reply to Re^3: Autovivification sucking the life out of me
in thread Autovivification sucking the life out of me

A simpler example perhaps:
use strict; use warnings; my %foo = ( BAZ => 1 ); sub see_args { }; warn sprintf "at start: BAR '%s'\n", ! exists $foo{BAR} ? '<absent>' : ! defined $foo{BAR} ? '<und +ef>' : $foo{BAR}; # see_args( @foo{qw(BAR BAZ)} ); my @a = @foo{qw(BAR BAZ)}; warn sprintf "at end: BAR '%s'\n", ! exists $foo{BAR} ? '<absent>' : ! defined $foo{BAR} ? '<und +ef>' : $foo{BAR};

Running this without the subroutine call, but rather the assignment to the array, results in output:

at start: BAR '<absent>' at end: BAR '<absent>'
That is, using the slice @foo{qw(BAR BAZ)} returns the values without modifying them.

Commenting out the assignment and instead executing the subroutine call produces this output:

at start: BAR '<absent>' at end: BAR '<undef>'
Somehow using the slice in the call to the subroutine has modified the values in the slice, creating an undef value for the key 'BAR'. How?