You could try doing a here-script type of thing:

  my $token = "__UNIQUE_TOKEN__";
  $token .= "_" while $str =~ /$token/;
  chomp (my $new_str = eval qq(<<"$token"\n$str\n$token));
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(I haven't tested that, but it feels like it might work). What I usually do for this type of thing is to not rely on lexical variables in my program, and instead set up a separate hash of legal variables:

   my %vars = ( var => "foo" );
   $str = 'abc $var def';
   (my $new_str = $str) =~ /\$(\w+)/$vars{$1}/g;
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