$to =~ s/\$(\d+)/$a[$1-1]/g;

##</code><code>##

$to =~ s/\\(.)|\${(\d+)})|\$(\d+)/
   (defined $1
      ? $1
      : (defined $2
           ? $a[$2-1]
           : $a[$3-1]
        )
   )
/eg;