The documentation for tr clearly states that it does not interpolate. As a workaround, we can interpolate into a string and then evaluate that string. The first two tests below demonstrate that we build the expected string and that it evaluates correctly. As a notational convenience, we can combine these two steps into one statement. The third test below demonstrates that form. 
use strict;
use warnings;
use Test::Simple tests=>3;

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

my $string = "\$bad =~ tr/$card//";
ok($string eq '$bad =~ tr/K//', "string = '$string'");

$kards = eval $string;
ok($kards == 5, "Number of K's in $bad is $kards"); 

$kards = eval "\$bad =~ tr/$card//";
ok($kards == 5, "Combined form");

[download]

OUTPUT:

1..3
ok 1 - string = '$bad =~ tr/K//'
ok 2 - Number of K's in AKQJT98765432KKKK is 5
ok 3 - Combined form

[download]
Bill

In reply to Re: tr operator in eval -- updated by BillKSmith
in thread tr operator in eval -- updated by pgmer6809

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