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?