C:\>perl -MO=Concise -e"@f = split ' ', shift"
9  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
8     <@> split[t5] vK ->9
3        </> pushre(/"\\s+"/ => @f) s*/64 ->4
6        <1> shift sK/1 ->7
5           <1> rv2av[t4] sKRM/1 ->6
4              <#> gv[*ARGV] s ->5
7        <$> const[IV 0] s ->8
-e syntax OK

C:\>perl -MO=Concise -e"@f = split /\s+/, shift"
9  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
8     <@> split[t5] vK ->9
3        </> pushre(/"\\s+"/ => @f) s/64 ->4
6        <1> shift sK/1 ->7
5           <1> rv2av[t4] sKRM/1 ->6
4              <#> gv[*ARGV] s ->5
7        <$> const[IV 0] s ->8
-e syntax OK

C:\>perl -MO=Concise -e"@f = split ?\s+?, shift"
9  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
8     <@> split[t5] vK ->9
3        </> pushre(/"\\s+"/ => @f) s/64 ->4
6        <1> shift sK/1 ->7
5           <1> rv2av[t4] sKRM/1 ->6
4              <#> gv[*ARGV] s ->5
7        <$> const[IV 0] s ->8
-e syntax OK