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Running and Debugging Perl - page 3 / 30

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Running and Debugging Perl

(Missing semicolon on previous line?)

In fact, this is exactly the problem:

print "Hello, world."

Line 7 doesn't end with a semicolon. But what of the error message, 'Scalar found where operator expected'? What does this mean? Like all of Perl's error messages, it means exactly what it says. Perl found a scalar where it thought there should be an operator. But why? Well, Perl had just finished processing a string, which was fed to print. But since there wasn't a semicolon, it was trying to find a way to continue the statement. The only way to continue would be to have an operator to link the string with something else: the concatenation operator, for instance, to connect it to another scalar. However, instead of such an operator, Perl found the scalar $a. Since you can't put a string right next to a variable, Perl complains, and as there's no way for this to make sense, it also gives us a 'syntax error'.

The next problem is in line 9:

if ($a == 1 {

Here we have no clue to help us track down the bug. It's a syntax error pure and simple, and we can fix it easily by providing the missing bracket. It should, of course, look like this:

if ($a == 1) {

Syntax Error Checklist

Tracking down syntax errors can be troublesome, but it's a skill that comes with practice. Most of the errors you're likely to experience are going to fall into one of the six categories below:

Missing Semicolons

We've seen this already, and it's probably the most common syntax error there is. Every statement in Perl, unless it's at the end of a block, should finish with a semicolon. Sometimes you'll get the helpful hint we got above:

(Missing semicolon on previous line?)

but otherwise you've just got to find it yourself. Remember that the line number you get in any error message may well not be the line number the problem occurs on – just when the problem is detected.

Missing Open/Close Brackets

The next most common error comes when you forget to open or close a bracket or brace. Missed closing braces are the most troublesome, because Perl sometimes goes right the way to the end of the file before reporting the problem. For example:

#!/usr/bin/perl

  • #

    braces.plx

use warnings; use strict;

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