You didn't specify, but if you're doing this on Linux you can use /proc/self/stat (see man proc for the details).

open my $stat,"<","/proc/self/stat" or die "Can't open /proc/self/stat
+"; 
my ($vsz,$rss)=(split ' ',<$stat>)[22,23];
print "$vsz - $rss\n"
[download]

Or you could use the ps command or Unix::Process