use strict;
my ($a,$b) = (shift, shift);
my @m = $a =~ /([^$b])/ig;
if ($#m < 0) {
    print "ok, can build '$a' using letters in '$b'\n";
    foreach my $letter (split // => $a) {
        unless ($b =~ s/$letter//) {
            print "but will need an extra '$letter'\n"
        };
    }
}
else {
    print "can't build '$a' because I don't have '",join("' and '", @m),"'\n";
}