#{
#  print "$_\n" for grep {$_} @a{keys %b};
#}
#
# should be something like (untested..)

foreach my $bkey (keys %b) {
    warn "key not defined" unless $bkey; ## what is the purpose of your "grep {$_}"???
    if ($a{$bkey}){print "FOUND: [$bkey] in the hash \%a\n"}
    else{print "NOT found key [$bkey] in the hash \%a\n"}
}