use strict;   # or die
use warnings; # or die

my %hash = (
   cogito   => "ergo sum"   # I think, so I exist
);

if (exists $hash{absum}) {
    print "Absum exists.";
} else {
    print "Absum doesn't exist.\n";
    # which would explain it's name...
}

print "Keys in the hash: ", join(" :: ", keys %hash), "\n";

__END__
Absum doesn't exist.
Keys in the hash: cogito