On Linux I'd rather use /proc/meminfo: it's fast and you obtain the total amount of mem/swap, too. A simplistic solution could be this:

use strict;
use warnings;

my $meminfo='/proc/meminfo';
my %Limits=('mem'=>0.99, 'swap'=>0.60);
my %Info=();

open(MEM,"<$meminfo") or die "Can't read $meminfo: $!";
while(defined($_=<MEM>)){
 if(/^(Mem|Swap):\s*/){
  my $which=lc($1);
  my ($total,$used)=split(/\s/,$');
  my $pct=$total?($used/$total):0;
  my $test=($pct ge $Limits{$which}?"WARN":"OK");
  $Info{$which}=[$used, $total, $pct, $test];
 }
}
close(MEM);

#Quick and dirty way to print the %Info hash in a readable form
use Data::Dumper;
print Dumper(\%Info);
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This way, the parsing code is pretty easy; after that, you can use %Info for anything you need.