To gain performance I would use those non-backtracking subpatterns "(?> )"
. From the Camels third edition (p.206):
"...if you're going to fail, it's best to fail quickly and get on with your life."

my @users;
foreach (@site) {
  / ^ (?>\s*) <!-- (?>\s+) USER (?>\s+) (?>\d+) (?>\s+) - (?>\s+) (\S+
+?) (?>\s+) --> (?>\s*) $ /ix
  and push @users, $1;
}
print @users;
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I've also specified the pattern as exactly as possible, because this will also fail earlier and thus speed up the engine.