perl - How to modify a single separator in between the text -

hello: looking modify 1 separator in between following text either awk, perl or sed

a,b,c,d,e,f,g,,,,k,l,m  

to output

a,b,c,d,e-f,g,,,,k,l,m 

how modify nth separator ? in advance

you can use sed,

sed 's/,/-/5' yourfile