The problem is your use of the stringifying comma (=>). The parser sees it following a bareword, and assumes that you want the string value "WISCONSIN" rather than the return value of the subroutine &WISCONSIN().

There are various ways around this: you can change the fat comma to a regular comma, or you can change the bareword into something Perl recognizes as subroutine call (since that's what a constant is actually implemented as).

my %capitol_map = (
    WISCONSIN , "Madison",
    ILLINOIS  , "Springfield",
    ...
);
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or

my %capitol_map = (
    &WISCONSIN => "Madison",
    ILLINOIS()  => "Springfield",
    ...
);
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Update: it has been suggested that both of these have the potential to confuse future readers of your code, which I find a strong enough argument to recommend against using this particular construct. Use the skinny comma or dws's suggestion above, instead. </update>

I was going to suggest %map = ( +WISCONSIN => "Madison"), but that doesn't actually do the trick with my perl. It does, however, work for getting keys back out of the hash, should you ever need to:

print $capitol_map{+WISCONSIN};
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If God had meant us to fly, he would *never* have given us the railroads.
    --Michael Flanders