#!/usr/bin/perl

use warnings;
use strict;
use POSIX qw(_exit);

foreach my $mod (qw(CPAN No::Such IO::Socket Not::Installed))
{
  printf "%20s: %-3s\n",$mod,is_module_installed($mod)?"Yes":"No";
}
[download]

sub is_module_installed
{
  my($mod)=@_;
  return (system("perl -M$mod -e 1 2>/dev/null") == 0);
}
[download]

sub is_module_installed
{
  my($mod)=@_;
  defined(my $pid = fork)
    or die "Fork error: $!\n";
  if ($pid)
  {
    # Parent
    waitpid $pid,0;
    return $? == 0;
  }
  else
  {
    # Child
    close(STDERR);
    eval "use $mod;";
    _exit($@?1:0);
  }
}
[download]

You could make it a little faster usingfork, to avoid firing up a new interpreter from scratch.

Updated: Tanktalus points out that the copy of perl that system('perl ...') will start may not be the same as the running one; fork will use the currently running interpreter in a new process, so avoids that problem. It should also be a bit faster.

Updated: Fixed Tanktalus' name in previous update. :) Also addressed his concerns about atexit using POSIX::_exit.