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sub mydump($) {
    print Data::Dumper->Dump( [ $_[0] ], ['*hash'] );
...
mydump( \%hash );
my $s2 = myjoin( ':', @hash{qw(key)} );
mydump( \%hash );

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