likbez:

Feature, per the tr docs Characters may be literals or any of the escape sequences accepted in double-quoted strings. But there is no interpolation, so "$" and "@" are treated as literals. A hyphen at the beginning or end, or preceded by a backslash is considered a literal. Escape sequence details are in the table near the beginning of this section.

So if you want to use a string to specify the values in a tr statement, you'll probably have to do it via a string eval:

$ cat foo.pl
use strict;
use warnings;

my $str1 = 'abcde';
my $str2 = 'eda';
my $diff1 = 0;

eval "\$diff1=\$str1=~tr/$str2//";

print "diff1: $diff1\n";

$ perl foo.pl
diff1: 3
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...roboticus

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