Erm, so just do it. You've said what you want, write the code.

my %hits;
while( <> ) {
  my( $key ) = /HIT:\s+(\S+)/;
  $hits{ $key }++;
}
my @distinct = keys %hits;
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Now you didn't say so, but if you want to preserve the order things appear in the input just push onto a list and use the hash to tell what you've already seen.

my @hits;
my %seen;
while( <> ) {
  my( $key ) = /HIT:\s+(\S+)/;
  next if $seen{ $key }++;
  push @hits, $key;
}
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