Use a hash of arrays. When populating them:

  push @{$testcenters{$o_city}, $o_tcenter;
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Then to sort them:

for my $city (sort keys %testcenters) {
  for my $center (sort @{$testcenters{$city}}) {
    print "$city: $testcenters<br>";
  }
}
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(I could make lots of comments about your variable names, using strict.pm, the way you've embedded HTML in your code, etc, etc, etc. But I'd wind up writing a book, so I'll just suggest reading Perl Best Practices and Code Complete instead.)