You need to post some code. Without it, everything's just a guess.

mod_perl doesn't deny access to shared memory. You can use GTop to get a process shared memory size and total size. For example,

#!/usr/bin/perl

use strict;
use warnings;
use GTop (); 

my $gtop = GTop->new;
my $proc_mem = $gtop->proc_mem(2872);
print "Shared memory of the current processes: ", 
my $share = $gtop->proc_mem($$)->share, "\n";

print "Total memory size of current processes: ",
my $size  = $gtop->proc_mem($$)->size, "\n";
[download]

For more help, see:
mod_perl intro
Measuring_the_Memory_of_the_Process

Update: This will get the shared memory and size for a single process:

#!perl

use strict;
use warnings;
use GTop ();
use Data::Dumper::Concise;

my $gtop = GTop->new(@ARGV);

my $pid = 2518;
my $proc_mem = $gtop->proc_mem($pid);
for ($pid) {
    warn Dumper
    my $share = $proc_mem->share,
    my $size  = $proc_mem->size;
}
[download]