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    %hash = ();
    foreach $_ (@array) {
      $hash{getkey($_)} = $_;
    }

  my %res = map {
    $_ => do {
...
        $re;
    } if !$res{$_};
  } @words;

  my %res;
  foreach my $word (@rw) {
...
      $re;
    }
  }

Benchmark: running foreach, map_it for at least 5 CPU seconds...
   foreach:  6 wallclock secs ( 5.34 usr +  0.00 sys =  5.34 CPU) @ 28
+6.87/s (n=1531)
    map_it:  5 wallclock secs ( 5.15 usr +  0.01 sys =  5.16 CPU) @ 13
+4.77/s (n=695)

  my @b = (7,5,5,6);
  my @a;
  @a = map { $_ > 5 ? $_ : $a[0] } @b;