This all depends on how much you are exactly storing in your hash tables, but this code should give you a hint, just run it and watch top or some other resource monitor:

#!perl -w
use strict;
$|=1;
my (%a,%b,%c);
for (0 .. 500_000) { # set higher if you dare
    $a{$_} = "x" x 60; # 60 character string
    $b{$_} = "x" x 60;
    $c{$_} = "x" x 60;
    print "\rrow $_" unless $_ % 10000;
}
[download]

note: this goes up to 190 mb for about 500_000 iterations on my machine YMMV.