[perl #131805] [REGRESSION] [PERF] .grep-ing with a code block is now almost twice slower ( .grep({/foo/}) )

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# New Ticket Created by  Aleks-Daniel Jakimenko-Aleksejev 
# Please include the string:  [perl #131805]
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See this benchable6 output: 
https://gist.github.com/Whateverable/6001c5744e3c877d76928d465de31e46

If you look closely, benchable has not only built the graph across all 
releases, but it also bisected the performance drop to the exact commit. Here 
is that commit: 
https://github.com/rakudo/rakudo/commit/b7201a8f22338a906f2d8027a21387e8f5c77f41

This was double-checked with committable6:
<AlexDaniel> c: b7201a8^,b7201a8 (^99999).grep({!/1/}).elems.say; say now - 
BEGIN now
<committable6> AlexDaniel, ¦b7201a8^: «59048␤5.0481256» ¦b7201a8: 
«59048␤9.0262629»

So it is ≈5 seconds before vs ≈9 seconds after. A significant performance drop 
even if we assume possible bench noise.

This was noticed because of this ticket: 
https://github.com/perl6/doc/issues/1425

See also IRC discussion: https://irclog.perlgeek.de/perl6/2017-07-27#i_14927626

And maybe coverable output on HEAD if it may help you: 
https://gist.github.com/Whateverable/f3db9e924cebe9b96a71a2b4cd67ae9c

Note that whether it's {!/1/} or {/1/} doesn't really matter: 
https://irclog.perlgeek.de/perl6/2017-07-27#i_14927789