use strict;
use warnings;

for (
    'oh {hello} there',
    'oh {hello there',
    'oh hello there',
    'oh hello} there',
) {
    print '',
     ( $_ =~ /
              (\{)?       # optional opening brace
              hello       # .. followed by 'hello'
              (?(1)\})    # a closing brace iif the open brace was there
             /x ? 'YEP ' : 'NOPE' ),
     " - $1\n";
}