sub mydump($) {
    print Data::Dumper->Dump( [ $_[0] ], ['*hash'] );
}

sub myjoin($@) {
    return join @_;
}

my %hash;
my $s1 = join( ':', @hash{qw(key)} );   # line no. 12
mydump( \%hash );
my $s2 = myjoin( ':', @hash{qw(key)} );
mydump( \%hash );

##</code><code>##

$ perl -w ./hashslices.pl
Use of uninitialized value within %hash in join or string at ./hashslices.pl line 12.
%hash = ();
%hash = (
          'key' => undef
        );