Just dereference the array in a scalar context:

#! perl
use strict;
use warnings;

my %families = (Flintstone => [ qw(Pebbles)                    ],
                Simpson    => [ qw(Bart Lisa Maggie)           ],
                Keaton     => [ qw(Alex Mallory Jennifer Andy) ]);

for my $name (keys %families)
{
    my  $num_children = @{ $families{$name} };   # <-- Here's where it
+ happens

    if ($num_children == 1)
    {
        print "There is $num_children child in the $name family\n";
    }
    else
    {
        print "There are $num_children children in the $name family\n"
+;
    }
}
[download]

Hope that helps,