Assuming you pipe the contents of a dict type file into the following script:

#! /usr/bin/perl

my %stem;
while( <> ) {
        chomp;
        next unless /^(.*)(?:m|rn)$/;
        print "${1}m -vs- ${1}rn\n" if ++$stem{$1} == 2;
}
[download]

... on FreeBSD 5.2 this produces the following:

album -vs- alburn
am -vs- arn
bam -vs- barn
bom -vs- born
bum -vs- burn
chum -vs- churn
cum -vs- curn
dam -vs- darn
dom -vs- dorn
dum -vs- durn
gam -vs- garn
gim -vs- girn
ham -vs- harn
hem -vs- hern
kim -vs- kirn
leam -vs- learn
scam -vs- scarn
sham -vs- sharn
stam -vs- starn
stem -vs- stern
swom -vs- sworn
tam -vs- tarn
tum -vs- turn
um -vs- urn
yam -vs- yarn
[download]

The only trouble is... I can't figure out which pair are the antonyms (mad props to halley :-). To tell the truth, I don't even know what a "swom" is...