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#!/usr/bin/perl
use strict;
...
  print("Evaluation error.\n"), next if not defined $result;
  print "Result: $result\n";
}

> 3*4.2e3+5^(3.1+2)
Entered: (3 * 4.2e3) + (5 ^ (3.1 + 2))
...
> a*b+c
Entered: (a * b) + c
Evaluation error.