Something like this? 

/@(?!bar\.).*\.example\.com$/

[download]
though that rejects "...@bar.foo.example.com" as well. This might be a good thing or a bad thing or a don't-care thing, you didn't specify.

So you could instead go for: 

/@(?!bar\.example\.com$).*\.example\.com$/

[download]

If your teacher will let you declare a variable, then you could even use: 

my $ex= ".example.com";
# ...
    if(  /@(?!bar\Q$ex\E$)\Q$ex\E$/  ) {

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or 
my $ex= quotemeta(".example.com");
# ...
    if(  /@(?!bar$ex$)$ex$/  ) {

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But all of these don't reject "...@foo.bar.example.com" which you might want to do, so you could go for: 
/@(.*\.)?(?!bar\.)[^.]+\.example\.com$/

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/@((.*\.)?(?!bar\.)[^.]+\.)?example\.com$/

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Updated based on shenme's reply.

                - tye

In reply to Re: Match all emails from all subdomains for a domain except one.. (tye) by tye
in thread Match all emails from all subdomains for a domain except one.. by ehdonhon

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