Use procedures. Here's a hypothetic example:

if (defined($max)) {
    $if = sub { ($Files >= $max) };
}
if (defined($min)) {
    if ($if) {
        my $oldif = $if; $if = sub { &$oldif() || ($IdleTime >= $min) 
+};
    } else {
        $if = sub { ($IdleTime >= $min) };
    }
}
# ...
# then you invoke it with
if (&$if()) {
    print "hi";
} else {
    print "hello";
}
[download]

And you may want to get a copy of MJD's HOP. I don't really know if it helps here, as I haven't yet got that book, but I think it may.

Update: don't belive what ikegami said above, eval-string is evil.

Update 2: but he's right about the boolean values, of course.