$ perl -MBenchmark=cmpthese -wE'my$x=join"    "=>"",("abc")x5,"";say"sourc: |$x|";sub splt{split" ",reverse((split" ",(reverse$x),1)[0]),1;};sub rgx{$x=~s/^\s+//r=~s/\s+$//r};say "split: |",splt(),"|";say"regex: |",rgx(),"|";cmpthese(-2,{splt=>\&splt,rgx=>\&rgx})'
sourc: |    abc    abc    abc    abc    abc    |
split: |abc    abc    abc    abc    abc|
regex: |abc    abc    abc    abc    abc|
          Rate  rgx splt
rgx  1047602/s   -- -64%
splt 2915089/s 178%   --