pfaut already answered your question, but when you have to grep through arrays to determine existance, you almost always should be using a hash and the exists function. Even if you need to keep them in the order that they were found, you can use your array in conjunction with a separate hash (or hash of hashes in your case):

push {@{$failed{$client}}, $sid unless exists $fail{$client}{$sid};
$fail{$client}{$sid} = undef;
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Or just do the second statement if you don't need to keep the keys in order, then it doesn't matter if it exists already.