Either I misunderstand the spec, or your solution fails: 

#!/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";
}

[download]

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.

In reply to Re^2: Multiline match for empty string by kennethk
in thread Multiline match for empty string by josh803316

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