In that case, how about this:

use Test::More;

my %tests
    = ( 'hello' => 1,
        '{hello}' => 1,
        'hello}' => 0,
        '{hello' => 0,
        'oh {hello} there' => 1,
        'oh {hello there' => 0,
        'oh hello there' => 1,
        'oh hello} there' => 0,
   );

plan 'tests' => scalar keys %tests;

while ( my ( $text, $result ) = each %tests ) {
    my $hello = qr/hello/;
    my $test_result
        = ( $text =~
            /
            \{$hello\}           # hello with braces
            |                    # or
            (?<! \{ )            # not a brace
            $hello               # hello
            (?!  \} )            # also not a brace
            /x
       ) ? 1 : 0;
    is( $test_result, $result, $text );
}
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Put your (possibly complicated) match text in a variable so you don't have to change it in two places when it changes. After that, literally match the text with braces and without.