Whole code looks like

while ($string=~m{<(ths|193)>|\.}g) {
            $count=$count+($tbsz * 0.25);
        }
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That makes a tr/// solution a non-candidate, doesn't it?

I'd probably go for something like:

my %factor;
$factor{ths} = $factor{193} = 0.25;
$factor{ems} = $factor{194} = 0.5;
$factor{ens} = $factor{195} = 1;
$factor{W} = $factor{' '} = $factor{"\n"} = $factor{"\t"} = ... = 1;
$factor{w} = $factor{'('} = $factor{')'} = 0.84375;
....
my $count;
while (/(?|<([A-Za-z0-9]+)>|(.))/g) {
    no warnings 'uninitialized';
    $count += $factor{$1};
}
$count *= $tbsz;
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You may want to consider replacing the (.) with ($charclass), where:

my $charclass = join "", grep {1 == length} keys %factor;
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Whether that makes a difference depends on your data set.