$ perl
my %h = 'a'..'f';

sub modify {
  for (my $i = 1 ; $i < @_ ; $i+=2) {
    $_[$i] .= '_modified';
  }
}

modify(%h);
print "$_ => $h{$_}\n" for sort keys %h;
^D
a => b_modified
c => d_modified
e => f_modified

$ perl
my %h = 'a'..'f';

# assignment to key (odd) positions has no side effects
$_ .= '_modified' for %h;
print "$_ => $h{$_}\n" for sort keys %h;
^D
[the same output]