Try this:

Win8 Strawberry 5.30.3.1 (64)  Sat 09/26/2020  0:05:59
C:\@Work\Perl\monks
>perl
use strict;
use warnings;

my $kards;
my $bad = 'AKQJT98765432KKKK';
my $card = 'K';

eval "\$kards = \$bad =~ tr/$card//";
print "1: Num of '$card' in $bad is == $kards \n";

my $kings = $bad =~ tr/K//;
print "2: Num of '$card' in $bad is == $kings \n";

__END__
1: Num of 'K' in AKQJT98765432KKKK is == 5
2: Num of 'K' in AKQJT98765432KKKK is == 5
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Note that  \$kards \$bad in the eval string are escaped (update: and $card is not). Why?

Update: Just read your update to the OP and I think you pretty much have the idea I was trying to get across. However, now try running the OP update code with strict and warnings enabled.