Other posters were correct when they changed things from using the {min,max} quantifier notation on the inside of the capture buffer to using the '?' quantifier on the outside. There is a crucial difference between "empty but matching" and "not matching", and {0,1} doesnt have the same behaviour as '?' even though they are functionally equivelent. (This may be construed as a bug :-) Alternatively you can do what i do below, which is to put the capture inside of an alternation.

Anyway, the following uses the conditional pattern with lookahead/lookbehind to match as you requested. You can play around with it to easily forbid {{hello}}, as the current code allows it.

use strict;
use warnings;

for (
    'oh {hello} there',
    'oh {hello there',
    'oh hello there',
    'oh hello} there',
    'of {{hello}} there',
) {
    if ( $_ =~ /
              (?:
               ( \{ )    # capture a '{'
              |          #   or
               (?<! \{ ) # not preceded by a '{'
              )
              hello      # .. followed by 'hello'
              (?(1)      # if (defined $1)
                \}       #     match an '}'
              |          # else
                (?! \} ) #     not followed by a '}'
              )          #
             /x) 
    {
        print "YEP : $_ : ",
              defined $1 ? "'$1'" : 'undef',
              " - $&\n";
    } else {
        print "NOPE: $_\n";
    }              
}

__END__
YEP : oh {hello} there : '{' - {hello}
NOPE: oh {hello there
YEP : oh hello there : undef - hello
NOPE: oh hello} there
YEP : of {{hello}} there : '{' - {hello}
[download]

Also I changed your diagnostic code, you were using $1 even when the match failed, which meant you were getting the wrong thing.