If you want to use tr with dynamic strings (which is NOT the case here), you need to use string eval. Be sure to only use it for validated strings, never a random user input! 
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
use utf8;

use open OUT => ':encoding(UTF-8)', ':std';

my $charset = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
my $boldset = '𝐚𝐛𝐜𝐝𝐞𝐟𝐠𝐡𝐢𝐣𝐤𝐥𝐦𝐧𝐨𝐩𝐪𝐫𝐬𝐭𝐮𝐯𝐰𝐱𝐲𝐳𝐀𝐁𝐂𝐃𝐄𝐅𝐆𝐇𝐈𝐉𝐊𝐋𝐌𝐍𝐎𝐏𝐐𝐑𝐒𝐓𝐔𝐕𝐖𝐗𝐘𝐙𝟎𝟏𝟐𝟑𝟒𝟓𝟔𝟕𝟖𝟗';

my $source = 'The quick brown fox jumps over the lazy dog 1234567890 times.';
my $target = $source;
eval "\$target =~ tr/$charset/$boldset/";

say for $source, $target;

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Re: Transform ASCII into UniCode by choroba
in thread Transform ASCII into UniCode by Perlian

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