From Perlfunc (slightly paraphrased): use Module; is exactly equivalent to BEGIN { require Module; import Module LIST; } except that Module must be a bareword. Also use Module (); is exactly equivalent to BEGIN { require Module }.

So just do the checks inside a BEGIN block and you shouldn't notice a difference.

In answer to your second question, I give you:

#!/usr/bin/perl
use strict;
use warnings;

if (1 == 2) {
  require "foo.pl";
}

print "bar\n";
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I don't have a foo.pl in my @INC path and this caused no errors or warnings, so I would say "no".