Ovid's solution works but keeps scanning the file even after [foo2] is found. It is no big deal if the foo file is small but if it is huge it might take a while. I added a flag to bail out once [foo2] is found.

#!/usr/bin/perl -w
use strict;

my $start = qr/\Q[foo]\E/;
my $end   = qr/\Q[foo2]\E/;
my $flag = 0;
while ( <DATA> ) {
    if (/$start/ .. /$end/) {
        $flag++;
        print;
        next;
    }
    last if $flag;
}
print "$flag lines printed\n";
__DATA__
;
; foo file
;
[foo]
a=1
b=2
c=3
[foo2]
d=4
e=5
f=6
[download]

--

flounder