sed -r "/^FOOBAR=.*$/d"      -i samefile &&
sed -r "$ aFOOBAR=newvalue" -i samefile

sed '/^FOOBAR=/{h;s/=.*/=newvalue/};${x;/^$/{s//FOOBAR=newvalue/;H};x}' infile

sed '/^FOOBAR=/{
h
s/=.*/=newvalue/
}
${
x
/^$/{
s//FOOBAR=newvalue/
H
}
x
}' infile

grep -q "^FOOBAR=" file && sed "s/^FOOBAR=.*/FOOBAR=newvalue/" -i file ||
    sed "$ aFOOBAR=newvalue" -i file

sed -n -e '/^FOOBAR=/!p' -e '$aFOOBAR=newvalue' infile

perl -ne '$c=1 if s/^FOOBAR=.*$/FOOBAR=newvalue/;
             print;
             END{print "FOBAR=newvalue" unless $c==1}' file > tmpfile &&
mv tmpfile file

awk -v varname="FOOBAR" -v newval="newvalue" '
    BEGIN {FS = OFS = "="}
    $1 == varname {$2 = newval; found = 1}
    {print}
    END {if (! found) {print varname, newval}}
' file > tempfile &&
mv tempfile file

grep -q '^FOOBAR=' somefile || echo 'FOOBAR=VALUE' >> somefile
sed -i 's/FOOBAR=.*$/FOOBAR=VALUE/' somefile

grep ^FOOBAR= my.file && perl -i -ple "s/^FOOBAR=.+/FOOBAR=newvalue/g" my.file || echo FOOBAR=newvalue >> my.file