use File::Find;           
my $size = 0;             
find(sub { $size += -s if -f $_ }, "foo");
[download]

That will recurse down into subdirectories. If that's not what you want, you could use:

use File::Find;           
my $size = 0;             
find(sub { $size += -s if $File::Find::name eq "foo/$_" and -f $_}, "f
+oo");
[download]

-sam