What I would do for something like this is have a limited number of operators, then scan for each and do the right thing. For example,

$operator = <STDIN>;
chomp $operator;
if ($operator eq '+')
{
   $result = $a + $b;
}
elsif ($operator eq '-')
{
   $result = $a - $b;
}
elsif ($operator eq '*')
{
   $result = $a * $b;
}
else
{
   die "unknown operator \"$operator\"\n";
}
[download]

I'm more comfortable with code like that because it leaves less room for unexpected errors from bad input.
But you can do what you're looking for with something like this, using eval:

$first=<STDIN>;
$second=<STDIN>;
$operator=<STDIN>;
chomp $first;
chomp $second;
chomp $operator;

$expression = "$first $operator $second";
print "$expression:\n";

$result = eval $expression;
if ($@)  # ($@ is set by eval if something went wrong)
{
    print "oh no!: $@\n";
}
else
{
    print "$result\n";
}
[download]

By the way, you shouldn't get into the habit of using $a and $b as variable names, since those two variables have special meaning for perl's sort function.

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.