#Assuming that $variable contains the value to match for 
#( Something from $runset{GROUND-NODE} ) . . .
$line =~ s/             #The substitution operator
           $variable    #vssa, vss, etc . . .
           :            #a colon
           .            #any character (I, O, etc)
          /             #substituted by . . .
          $variable     #as before
          :             #as before
          G             #the G because it's in GROUND-NODE
          /xg;          #Substitute globally

##</code><code>##

#Complete code!
#(Almost . . .)
# @lines contains the *PIN lines . . .

foreach $PIN (@lines) {
    foreach $ground_var (split /,/, $runset{GROUND-NODE}) {
        $PIN =~ s/$ground_var:./$ground_var:G/g;
    }
    foreach $power_var (split /,/, $runset{POWER-NODE}) {
        $PIN =~ s/$power_var:./$power_var:P/g
}