Although choroba already warned against using eval - and he is certainly right with it -, you can consider an eval-solution, if you understand the risks (and, in particular, if you have control over what goes into your file).

In this case, you could do:

use strict; use warnings;
while(my $line=<FH>) {
  print(eval "qq($line)");
}
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This solution is much simpler than doing a substitution manually, but never forget: Whoever creates file.txt, can get *any* code being executed by your program; it requires that every variable referenced in the file, exists in the scope of evaluation (and will be replaced); and finally, that, if file.txt contains parentheses, that they are balanced.