$ cat test.csv
foo,bar,baz,qux
1,a,AA,Fubberwurmenslijm
2,b,BB,Keveroog
3,c,CC,Boomslanghuid

$ perl -MText::CSV_XS=csv -wE'csv(in=>csv(in=>"test.csv",bom=>1,on_in=>sub{$_{merged}=$_{bar}.$_{baz}}))'
qux,bar,merged,foo,baz
Fubberwurmenslijm,a,aAA,1,AA
Keveroog,b,bBB,2,BB
Boomslanghuid,c,cCC,3,CC

$ perl -MText::CSV_XS=csv -wE'csv(in=>"test.csv",bom=>1,headers=>[qw(foo bar baz qux merged)],on_in=>sub{$_{merged}=$_{bar}.$_{baz}})'
foo,bar,baz,qux,merged
1,a,AA,Fubberwurmenslijm,aAA
2,b,BB,Keveroog,bBB
3,c,CC,Boomslanghuid,cCC