Check the rand function used by your version of Perl. Specifically, check for the number of randbits it is configured for:

P:\test>Perl -v

This is perl, v5.8.7 built for MSWin32-x86-multi-thread
(with 7 registered patches, see perl -V for more detail)
...

P:\test>Perl -V:randbits
randbits='15';
[download]

Which means that with AS perl, this snippet will only ever produce 32768 unique ids:

#! perl -slw
use strict;

my %cache;

$cache{ sprintf("%0.8x",rand()*0xffffffff) } = undef for 1 .. 1_000_00
+0;

printf "1 million attempts produced %d keys\n", scalar keys %cache;
printf "between %s and %s\n", (sort keys %cache)[ 0, -1 ];
__END__
P:\test>junk
1 million attempts produced 32768 keys
between 00000000 and fffdffff
[download]

I'm not sure what the mathematical probability is for picking duplicates from a pool 2^15 values, but in practice, it produces very, very few unique values before it repeats itself:

#! perl -slw
use strict;

my %cache;
my $u = sprintf("%0.8x",rand()*0xffffffff);

until( exists $cache{ $u } ) {
    $cache{ $u } = undef;
    $u = sprintf("%0.8x",rand()*0xffffffff);
}

printf "Produced %d unique keys before duplicating\n", scalar keys %ca
+che;
printf "between %s and %s\n", (sort keys %cache)[ 0, -1 ];
__END__
P:\test>junk
Produced 454 unique keys before duplicating
between 00bdffff and ff89ffff

P:\test>junk
Produced 273 unique keys before duplicating
between 013dffff and feb5ffff

P:\test>junk
Produced 84 unique keys before duplicating
between 0183ffff and fe5bffff

P:\test>junk
Produced 94 unique keys before duplicating
between 001bffff and fbc1ffff
[download]

You can extend the range almost linearly by multiplying two calls to rand:

#! perl -slw
use strict;

my %cache;

$cache{ sprintf( "%0.8x", rand() * rand() * 0xffffffff ) } = undef for
+ 1 .. 1_000_000;

printf "1 million attempts produced %d keys\n", scalar keys %cache;
printf "between %s and %s\n", (sort keys %cache)[ 0, -1 ];
__END__
P:\test>junk
1 million attempts produced 994758 keys
between 00000000 and ffae0137
[download]

but even then, the number of uniques you will get before repeats is dismally low:

Produced 17891 unique keys before duplicating
between 00000000 and fba0116f

P:\test>junk
Produced 1957 unique keys before duplicating
between 0008e633 and fb575407

P:\test>junk
Produced 30229 unique keys before duplicating
between 00000000 and fdeaed07

P:\test>junk
Produced 10830 unique keys before duplicating
between 0000eedf and ff441eff

P:\test>junk
Produced 15537 unique keys before duplicating
between 00000000 and fe068a77
[download]

All in all, a better session id generator than this would be strongly advisable.