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
[download]

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
[download]

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;
[download]

Or one could, as toolic suggests, actually look at perldata and search for the string '$#', which yields

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

as the second hit, followed by a few more informative paragraphs.

---

Give a man a fish:  <%-(-(-(-<

Or one could, as toolic suggests, actually look at perldata and search for the string '$#', which yields

Hmm, an exercise in using available search methods ... to see the available ways one could stumble upon the same information toolic offered ... you understand this is what I presented right ? A different path to the information toolic presented?