I think that writing this out will turn out to be the most efficient method, except that you only need to get the substr once for your example:

while(<IN>)
{
    my $t = substr($_, 259, 1);
    if($t eq "B")
    {
        print B $_;
    }
    else if ($t eq "T")
    {
        print T $_;
    }
    else if ()
    {
        # and so on for at least 10 lines!...
    }
}
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-- 
Joost       downtime n. The period during which a system
            is error-free and immune from user input.
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