my $arrayref = [ 1, 2, 3 ];

#step-by-step
my @array = @$arrayref; # derefernce to array, @{ $arrayref } works too
my $num_elements = @array;

#in one step
$num_elements = @$arrayref;

#or, when in list context
print scalar @$arrayref;

#or, if it's the last element number you want
print $#$arrayref;