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

my $tests = '% only 1 profile is allowed

';

my $regex = '\S';
if($tests !~ qr{$regex}ms) {
  print "$tests\n REGEX MATCHED\n";
} else {
  print "$tests\n FAILED!\n";
}
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You could also dynamically build your regular expression based upon the string itself:

my $regex = '^\n{' . $tests =~ tr/\n/\n/ . '}';

If neither of these meet spec, I'd appreciate a more thorough description of what you mean by "maintain this multiline match regex format". I sense this is an XY Problem.

What I mean is, in the flow of the application I can only pass a regex but the logic of the comparison won't change:
$tests =~ qr{$regex}ms

So I was wondering if there was a regex I could pass besides ^$ that would fail the match for anything that wasn't a single empty space ''.

It's probably easier and cleaner to just write a one-off test rather than working within this fixed framework, but you could use this to check if your test string contains only whitespace:

my $regex = q|(??{'\s{' . length($tests) . '}'})|;

This requires you to include use re 'eval'; at some point in scope. It also requires that the name of of the variable being tested is literally $tests - if not, you need to modify the string accordingly.

How it works:
The expression uses Perl code executed at matching time - see (??{ code }) in perlre. The string literal stored in $regex is (??{'\s{' . length($tests) . '}'}). When the regular expression is executed, Perl concatenates '\s{', the length of the variable $tests and '}'. The resulting regular expression requires that the string in question match exactly as many whitespace characters as there are characters in the string, i.e. contain only whitespace. You should also note that this opens up some security holes in Perl, as discussed in (??{ code }) and in A bit of magic: executing Perl code in a regular expression from perlretut.

What?

An empty string can't contain a newline

Why not just do a quick scan for two newlines in a row before you go and do the main regex?

"(?!.*[^\n])" or "(?!.*\\S)" depending on how strict you want to be.

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.

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.