Here are some simple Unix and Windows one-liners that check whether a module is installed. They return 1 or 0 accordingly:

    (Unix) perl -le 'print 0 + eval "require $ARGV[0]"'   Some::Module

    (Win)  perl -le "print 0 + eval qq(require $ARGV[0])" Some::Module
[download]

Or as a program with clearer output:

    #!/usr/bin/perl -wl

    for my $module (@ARGV) {
        print "$module is ", 
              eval "require $module" ? "" : "not ",
              "installed.";
    }

    __END__


    $ perl installed.pl Parse::RecDescent Parse::RecklessDescent

    Parse::RecDescent is installed.
    Parse::RecklessDescent is not installed.
[download]

--
John.