sub by_hashval_func {
    my ( $hashref ) = @_;
    return sub { $hashref->{ $a } cmp $hashref->{ $b } };
}

foreach my $key ( sort by_hashval_func( \%hash ) keys %hash ) {
    # do something useful
}

##</code><code>##

sub by_hashval_func {
    my ( $hashref ) = @_;
    return sub($$) { $hashref->{ $_[0] } cmp $hashref->{ $_[1] } };
}