in reply to what is the use of derefernce in substitution?

In that example there is no difference between ${1} or ${2} and $1 or $2.

It could just as well be written as: 

 $text =~ s/(<jrnl;\d+;)[^>]*(>)/$1$annualIssue;$first\\N$last$2/gs;

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In Section Seekers of Perl Wisdom