I don't think your code works. When I run it for input 
ssDDssUUss

[download]

it returns 

OOMMIIMMss

[download]

while you previously claimed the correct output is 

OOMMIIMMOO

[download]

Similarly, 

ssUUssDDss

[download]

is transformed into

IIMMOOMMss

[download]

while the correct output should be

IIMMOOMMII

[download]

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

In reply to Re^6: Pattern matching simultaneous substitution by choroba
in thread Pattern matching simultaneous substitution by Anonymous Monk

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