I'd throw the answers in a hash: for a typical logfile, you'll get a lot of the same answers anyway:

open(LOGFILE, "$mylogfile") or die "Could not open $mylogfile: $!\n";
my %useragent;
while(<LOGFILE>) {
  if (/Mozilla(.*)$/) {
    $useragent{$1}++;
  }
}
close(LOGFILE);

## Alphabetically sorted:
for (sort keys %useragent) {
  printf "Length: %3d Frequency: %5d Name: $_\n", length $_, $useragen
+t{$_};
}

## Sorted by frequency:
for (sort {$useragent{$a} <=> $useragent{$b}} keys %useragent) {
  printf "Length: %3d Frequency: %5d Name: $_\n", length $_, $useragen
+t{$_};
}

## Sorted by length:
for (map  { $_->[0] }
     sort { $a->[1] <=> $b->[1] }
     map  { [ $_, length $_ ] }
     keys %useragent) {
  printf "Length: %3d Frequency: %5d Name: $_\n", length $_, $useragen
+t{$_};
}
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