Here's the fastest solution under linux. Note that sysinfo(2) says that since Linux 2.3.23 (i386), 2.3.48 (all architectures) the results are returned in $s[13] bytes units, not bytes. I, however, have Linux 2.4.25-gentoo here, and it returns the data in bytes so I assume that convention. In the following code, $t is the total memory, $f is the amount of free memory, not including swap or himem (which is the memory over 4G(?) on 32-bit systems).

sub __NR_sysinfo(){116} syscall(__NR_sysinfo, $s = pack("x256")) and d
+ie $!; @s = unpack("l!L!9S!L!2I!", $s); ($t, $f) = @s[4, 5]; printf "
+%.0fK used memory\n", ($t-$f)/1024;
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#!/usr/bin/perl
use Linux::MemInfo;

%hash = get_mem_info();

foreach(sort keys %hash) {
    print "$_ = $hash{$_} \n";
}
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You'll have to decide whether you want to include swap usage in it or not.

In linux, parse /proc/meminfo. In other unix systems, you might try to parse the output of free, or top bn1 if free doesn't work for some reason. (Update: it seems that free doesn't exist on solaris. top does work, but its arguments are different from the linux version.)

use strict;
use Win32::OLE qw(in);

my $WMI = Win32::OLE->GetObject("winmgmts:")
    or die ("Could not create WMI object.");

my $OS_Col = $WMI->InstancesOf('Win32_OperatingSystem')
    or die ("Could not get OS instances.");

foreach my $OS (in $OS_Col) {
    printf "Free Physical Memory:  %10u kB\n", $OS->{FreePhysicalMemor
+y};
    printf "Free Virtual Memory:   %10u kB\n", $OS->{FreeVirtualMemory
+};
    printf "Total Virtual Memory:  %10u kB\n", $OS->{TotalVirtualMemor
+ySize};
    printf "Total Visible Memory:  %10u kB\n", $OS->{TotalVisibleMemor
+ySize};
}
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