The constant you are assigning is WISCONSIN to WI not WI to WISCONSIN. That said there is also a difference between WI and "WI" as

use constant WI => "WISCONSIN";
use constant ILLINOIS => "IL";

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

my $abbrev = WI;

if ( exists $capitol_map{$abbrev} ) {
    print "capitol = $capitol_map{$abbrev}\n";
}
else { print "False" }
[download]

will print the name of the capitol, and

use constant WI => "WISCONSIN";
use constant ILLINOIS => "IL";

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

#THE LINE BELOW WAS CHANGED
my $abbrev = "WI";

if ( exists $capitol_map{$abbrev} ) {
    print "capitol = $capitol_map{$abbrev}\n";
}
else { print "False" }
[download]

will not.

update:Hrm.. maybe I missed the fact that the OP might have wanted the WISCONSIN in the hash to become WI not the WI in the my $abbrev to become WISCONSIN ... so my answer would then be incorrect as i seem to be answering the wrong thing.

-enlil