1. If $REFERRERS is not yet defined, it will automagically become a hashref ... [emphasis added]

A small but, IMO, important point: In the case cited (and if strictures were not enabled; see strict), a hash named  %REFERRERS of the package global flavor would be autovivified, not a hash reference. A hash reference would be created if the  -> operator were used (see perlop).

c:\@Work\Perl>perl -w -MData::Dump -le
"$REFERRERS{foo}{bar}{baz}++;
 ;;
 dd \%REFERRERS;
"
{ foo => { bar => { baz => 1 } } }

c:\@Work\Perl>perl -w -MData::Dump -le
"$REFERRERS->{foo}{bar}{baz}++;
 ;;
 dd $REFERRERS;
"
{ foo => { bar => { baz => 1 } } }
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