Kolakoski

Score: 65.19 (pass)

#!perl -l
$_.=($d=$ARGV[$|--])x(/./g,$d)[$f++]until/.{$ARGV[2]}/;print$&

Score: 66.20 (pass)

#!perl -l
$c=pop;$_.=($d=$ARGV[$|--])x(/./g,$d)[$f++]until/.{$c}/;print$&

Score: 68.24 (pass)

somewhat better :-)

#!perl -l
$c='.'x pop;$_.=($d=$ARGV[$f%2])x(/./g,$d)[$f++]until/$c/;print$&

Score: 73.24 (pass)

#!perl -l
$c='.'x pop;$d=$ARGV[$f%2],$_.=$d x((/./g)[$f++]||$d)until/$c/;print$&

Score: 78.25 (pass)

attempt #2 : slightly better

#!perl -l
$c='.'x pop;$d=$ARGV[$f%2],$_.=$d x(substr($_,$f++,1)||$d)until/$c/;print$&

Score: 94.23 (pass)

first shot

#!perl -l
($n,$d)=@ARGV;$l='.'x pop;$_.=$n x(/$c(.)/?$1:$n),$c.='.',($d,$n)=($n,$d)until/$l/;print$&;