OK, you’re wrong. SCNR. :-)

First off, for the reason given by QM, this regex doesn’t prevent a match if there is a { character between } characters:

 2:57 >perl -wE "my $s = '}abc{def}'; say 'match' if $s =~ /}.*(?!{)}/
+;"
match

 2:59 >
[download]

Second, there is no non-greedy match here: .* is greedy. But making it non-greedy doesn’t help:

 2:59 >perl -wE "my $s = '}abc{def}'; say 'match' if $s =~ /}.*?(?!{)}
+/;"
match

 2:59 >
[download]

A negated character class is what is needed:

 3:31 >perl -wE "my $s = '}abc{def}'; say 'match' if $s =~ /}[^{]*?}/;
+"

 3:31 >perl -wE "my $s = '}abc|def}'; say 'match' if $s =~ /}[^{]*?}/;
+"
match

 3:31 >
[download]

Hope that helps,

Update: Tweaked the third code snippet.