You need to compile the Perl code in $if, which is what eval EXPR does.

if (eval $if) {
    print "hi";
} else {
    print "hello";
}
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But in the example you've shown, you're better off just storing a boolean value in $if:

$min = some number if defined
$max = some number if defined
$Files = some number 
$IdleTime = some number 

$if = 0;

$if = ($Files >= $max)
   if defined $max;

$if ||= ($IdleTime >= $min)
   if defined $min;

if ($if) {
    print "hi";
} else {
    print "hello";
}
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Update: When using the eval method, you probably want to use single quotes when building up $if. Let eval do the interpolation.