Sounds like a job for GTop.pm, which is a Perl interface to libgtop. I haven't used it myself, but it looks like you can write something to this effect:

#!/usr/bin/perl -w

use strict;
use GTop ();

my $gtop = GTop->new;
my $mem = $gtop->mem;

my $total = $mem->total;
my $free  = $mem->free;

if ($free / $total > .5) # Or whatever
{
    # If there's enough free memory, execute your
    # code, etc.
}
[download]

-Matt