I tried it. No luck. Very strange. I've used make -Dvariable in the past with no troubles, but nothing I did would change its mind about perl being in /usr/bin/perl. I even did make -DPERL=/usr/local/bin/perl -VPERL (as well as -E) and it would output /usr/bin/perl
Anyway, I finally just cheated and edited the /usr/ports/Mk/bsd.port.mk file temporarily and that worked.