our %ESCAPES = (
   n => "\n",
   t => "\t",
);

my $interpolated = '';

{
   local *_ = \$string;  # Alias $_ to $string.

   for (;;) {
      if (/\G \$(\w+) /gcsx || /\G \${(\w+)} /gcsx) {
         $interpolated .= eval '$'.$1;
         next;
      }

      if (/\G \\(.) /gcsx) {
         $interpolated .= exists($ESCAPES{$1}) ? $ESCAPES{$1} : $1;
         next;
      }

      /\G
         (
            .            # Catchall.

            (?:          # These four lines are optional.
               (?!\\)    # They are here to speed things up
               (?!\$)    # by avoiding adding individual
            .)*          # characters to the $interpolated.
         )
      /gcsx && do { $interpolated .= $1; next; };

      last;
   }
}

##</code><code>##

our %ESCAPES = (
   n => "\n",
   t => "\t",
);

sub interpolate {
   local *_ = \$_[0];  # Alias $_ to $_[0].

   my $interpolated = '';

   for (;;) {
      if (/\G \$(\w+) /gcsx || /\G \${(\w+)} /gcsx) {
         no strict 'refs';
        #no warnings;
         $interpolated .= ${$1};
         next;
      }

      if (/\G \\(.) /gcsx) {
         $interpolated .= exists($ESCAPES{$1}) ? $ESCAPES{$1} : $1;
         next;
      }

      /\G
         (
            .            # Catchall.

            (?:          # These four lines are optional.
               (?!\\)    # They are here to speed things up
               (?!\$)    # by avoiding adding individual
            .)*          # characters to the $interpolated.
         )
      /gcsx && do { $interpolated .= $1; next; };

      last;
   }

   return $interpolated;
}

##</code><code>##

our %ESCAPES = (
   n => "\n",
   t => "\t",
);

sub interpolate {
   local *_ = \$_[0];  # Alias $_ to $_[0].
   my $symtab = $_[1];

   my $interpolated = '';

   for (;;) {
      if (/\G \$(\w+) /gcsx || /\G \${(\w+)} /gcsx) {
         if (!exists($symtab->{$1})) {
            $interpolated .= "[unknown symbol \$$1]";
         } elsif (!defined($symtab->{$1})) {
            $interpolated .= "[undefined symbol \$$1]";
         } else {
            $interpolated .= $symtab->{$1};
         }
         next;
      }

      if (/\G \\(.) /gcsx) {
         $interpolated .= exists($ESCAPES{$1}) ? $ESCAPES{$1} : $1;
         next;
      }

      /\G
         (
            .            # Catchall.

            (?:          # These four lines are optional.
               (?!\\)    # They are here to speed things up
               (?!\$)    # by avoiding adding individual
            .)*          # characters to the $interpolated.
         )
      /gcsx && do { $interpolated .= $1; next; };

      last;
   }

   return $interpolated;
}

my %symtab = (
   string => "cheese",
  #user   => $user,
  #...
);
my $need_to_interpolate = 'smell my $string\n';
print interpolate($need_to_interpolate, \%symtab);