a comment rather than an answer. Consider:

use strict;
use warnings;
use Encode;

binmode *STDOUT, 'utf8'; # Suppress "wide character" warnings

my $CharSet = 'a'; # ASCII
my $BoldSet = pack('U', 119834); # Unicode bold 'a'

my $Source = 'a';
my $trTarget = $Source;
my $reTarget = $Source;

$trTarget =~ tr/$CharSet/$BoldSet/;
$reTarget =~ s/$CharSet/$BoldSet/;

print "$Source\n$trTarget\n$reTarget\n";
print $BoldSet;
[download]

Prints:

a
l
𝐚
𝐚
[download]

It seems tr/// isn't the right tool for the job. :-(

Update: PerlMonks is screwing up the unicode characters. They render correctly when I paste them into the edit window, but are shown as code points when I submit the edit. Bugger.