my @list = ( { a => 1 }, { a => 15 } );

# here the values of a in the hashrefs are changed, ok!
work_( $_ ) for @list;
print "_=".(ref $_ ? "r.$_->{a}" : $_) . "\n" for @list;

# but here the hashrefs in @list are replaced by scalars,
# namely the result of the work_() call.
work_( $_, 1 ) for @list;
print "_=".(ref $_ ? "r.$_->{a}" : $_) . "\n" for @list;

sub work_ {
	my ($obj, $act) = @_;
	$obj->{a}++;
	$_ = $obj->{a} if $act;
}