my %list;
$list{foo} = [1, 2, 3];
$list{bar} = [4, 5, 6];

my $src = 'foo';
my @list = @{$list{$src}};
print 'length = ', scalar @list, "\n";

# or...

$src = 'bar';
print 'length = ', scalar @{$list{$src}}, "\n";