The documentation explains it: perldata ... search for $#

Let see what all explain it [perldoc://"$#"] "$#" -> perlref   $aref->$#*; # same as $#{ $aref }

Wow, what a horrible search engine, that it five pages of results

 [ddg://"$#" site:perldoc.perl.org] "$#" site:perldoc.perl.org -> more of same nonsense, wow

hello http://grep.cpan.me/?q=\%24%23\w%2B+dist%3Aperl+ ->

The Perl equivalent for this is C<$#myarray>.

Set the highest index in the array to the given number, equivalent to
Perl's C<$#array = $fill;>.

RJ
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hello http://grep.cpan.me/?q=\%24%23\w%2B%20dist%3Aperl%20+dist=perl

trigger for I<container magics>, i.e. it will for C<%ENV> or C<%SIG>
but not for C<$#array>.

perl-5.22.0/pod/perldata.pod

    $days{'Feb'}    # the 'Feb' value from hash %days
    $#days        # the last index of array @days

The length of an array is a scalar value.  You may find the length
of array @days by evaluating C<$#days>, as in B<csh>.  However, this
isn't the length of the array; it's the subscript of the last elemen

which is a different value since there is ordinarily a 0th element.
Assigning to C<$#days> actually changes the length of the array.
Shortening an array this way destroys intervening values
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hello http://search.cpan.org/grep?cpanid=RJBS&release=perl-5.22.0&string=\%24%23&i=1&n=1&C=0#pod/perldata.pod

pod/perldata.pod

53:    $#days           # the last index of array @days
357:of array @days by evaluating C<$#days>, as in B<csh>.  However, th
+is
360:Assigning to C<$#days> actually changes the length of the array.
364:X<$#> X<array, length>
373:    $#whatever = -1;
382:    scalar(@whatever) == $#whatever + 1;
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