Some of the ones I can remember off the top of my head...

# slurp a bunch of files
@files = { local(@ARGV, $/) = @filenames; <> };

# get the indexes of items in an array
@foo{ @foo } = 0 .. @foo;
$foo{'abc'} # the index of 'abc' in @foo

# Get the sign of a number
$a <=> 0

# test primality
sub isPrime { (1 x $_[0]) !~ /^(11+)\1+$/ }
# (ok I never use that in production code
#   but it is funny)
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Ted