perl -Dt -e '"$a$b".$c.$d'   

EXECUTING...

(-e:0)	enter
(-e:0)	nextstate
(-e:1)	gvsv(main::a)
(-e:1)	gvsv(main::b)
(-e:1)	concat
(-e:1)	gvsv(main::c)
(-e:1)	concat
(-e:1)	gvsv(main::d)
(-e:1)	concat
(-e:1)	leave