Noting a point which hasn't come up yet:

which I assumed, in accordance with perldoc, would roughly translate to:

my %res;
foreach my $word (@rw) {
  $res{$word} ||= do {
    my $re;
    $re .= $translations{$_} for split //,$word;
    $re;
  }
}
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No, it doesn't. You are using || here; that shortcircuits, ie if $res{$word} already exists (and is true), then the do {} won't even execute. There's no way to duplicate this with map.

Btw, this:

my $re;
$re .= $translations{$_} for split //,$word;
$re;
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is more succintly expressed as

join '', @translations{ split //, $word };
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Makeshifts last the longest.