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use Benchmark;

...
 grep_seen:     15 wallclock secs (14.86 usr +  0.01 sys = 14.87 CPU) 
+@ 67.23/s (n=1000)
keys_map_1:     50 wallclock secs (46.78 usr +  0.83 sys = 47.61 CPU) 
+@ 21.00/s (n=1000)
keys_map_undef: 43 wallclock secs (42.16 usr +  0.94 sys = 43.09 CPU) 
+@ 23.21/s (n=1000)

my @b = map {$a[$_] eq $a[$_ + 1] ? () : $a[$_]} 0..$#a;