When allowing the emdedding of code (loosely defined), provide an escape mechanism!!! For example,

• I have no means of replacing with $1 . "00". Perl uses "${1}00".
• I have no means of replacing with '$1.00'. Perl uses the literal "\$1.00".

When adding your escape mechanism, careful not to break existing functionality. For example,

• Continue allowing me to replace with '\$1'. Perl uses "\\$1".

Update: A solution is to replace

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

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