I actually have a collection of favorites depending on context. One of my perennial favorites is:

# hand wavium prior to this line
my %een=();   # keep track of values we have already seen
@rray = grep !$een{$_}++ sort @rray;
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actually there are two in there. I frequently use @rray for a throw away array.

Others include $tring, $thing, $that (extra $this),$tatus and many more.