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

my @tests = ("\% only 1 profile is allowed\n\n",  # Should fail
             "\n\n",                              # Should pass
             "",                                  # Should pass
             );

my $regex = "(?!.*\\S)";
for (@tests) {
    print $_ =~ qr{$regex}ms ? "$_\n REGEX MATCHED\n" : "$_\n FAILED!\
+n";
}
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Like with the bad OP proposal, your regex passes for all cases.

Update: Fixed mistake in posted test, though it does not change the result. See below for corrected regular expression.

I forgot to anchor. Should be

"\\A(?!.*[^\n])" or "\\A(?!.*\\S)"

Of course, the following are much simpler:

"\\A[^\n]*\\z" or "\\A\\s*\\z"

By the way, your test is wrong. You never actually attempt to match.