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sub by_hashval_func {
    my ( $hashref ) = @_;
...
foreach my $key ( sort by_hashval_func( \%hash ) keys %hash ) {
    # do something useful
}

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