Declaring variables in Perl

Question

I'm learning Perl, and facing some inconsistency between running a program from the command line versus interactively executing it interactively in the debugger.

Specifically, I invoke the Perl debugger with perl -d -e 1, and run this code line-by-line

my $a = 1;
print $a;

$b = 2;
print $b;

In the output I am only seeing the value of $b, while $a seems to be undefined. At the same time, when I execute the same statements with perl myscript.pl, both values are shown in the output. Why does this happen? What am I missing?

Answer 1

The debugger is a wholly different environment from run time Perl. Each line you enter behaves like a separate block, and if you declare a lexical variable like my $a then it will be deleted immediately after the command.

It is as if you had written

{ my $a = 1; }
{ print $a; }

{ $b = 2; }
{ print $b; }

Ordinarily you will declare lexical variables at an appropriate point in the program so that they don't disappear before you need them. But if you want to use the debugger to play with the language then you need to use only package variables, which never disappear and are what you get by default if you don't use my.

Command line "one-liner" Perl programs usually do the same thing, but it's a lesson you will have to unlearn when you come to writing proper Perl programs. You will be using use strict and use warnings at the head of every program, and strict requires that you choose between lexical or package variables by using my or our respectively. If you try to use a variable that you haven't previously declared then your program won't compile.

Also never use $a or $b in your code. Apart from being dreadful variable names, they are reserved for use by the sort operator.

I hope that helps.