pack doesn't support this directly. This will do the job, though it can probably be made significantly more efficient:

sub toComp3{ 
    my $num = shift; 
    my @digits = split'',abs $num; 
    unshift @digits, 0 unless @digits & 1;
    push @digits, $num < 0 ? 13: 12; 
    my $comp3 = ''; $comp3 .= chr( shift( @digits ) * 16 + shift( @dig
+its ) ) 
        while @digits; 
    return $comp3 
}

print unpack 'H*', toComp3( 240 );;
240c
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It also doesn't cover unsigned numbers. Though a flag to indicate signed-ness could be added.