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

at start: BAR '<absent>'
at end: BAR '<absent>'
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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>'
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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?