It might be noteworthy that Conway's Perl Best Practices recommends against constant, partly for exactly the reason you're having trouble (they don't interpolate) - a similar issue arises when they're used in hashes, as documented in "Caveats" in constant.
The suggested alternative is Readonly, but it has a few bugs, and I've found Readonly is not really necessary if you just stick to not modifying variables whose names are in $ALL_CAPS (people with a more defensive coding philosophy might disagree).
Of course you are free to disagree with PBP on this point and use constant, in which case I think the solutions given by AnomalousMonk and BrowserUk above are preferable to the following, because they catch errors due to typos in the constant names better. But since this is Perl, and in the spirit of TIMTOWTDI, here's another solution:
Fields are separated with the OUT_FIELDSEP character. Enter a value
between VAL_MIN and VAL_MAX.
qw/ OUT_FIELDSEP VAL_MIN VAL_MAX /;