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s/([A-Z0-9]{5,})/$seenit{$1}++ or print "$1\n"/egi while <>;

s/([A-Z0-9]{5,})/$seenit{$1}++/egi while <>;

...
for (sort {$seenit{$a} <=> $seenit{$b} or $a cmp $b} keys %seenit) {
  print "$_: $seenit{$_}\n";
}