perldoc strict: strict vars

This generates a compile-time error if you access a variable that was neither explicitly declared (using any of my, our, state, or use vars ) nor fully qualified. (Because this is to avoid variable suicide problems and subtle dynamic scoping issues, a merely local variable isn't good enough.) See my, our, state, local, and vars.

Code: (dl )

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    use strict 'vars';
    $X::foo = 1;	# ok, fully qualified
    my $foo = 10;	# ok, my() var
    local $baz = 9;	# blows up, $baz not declared before
    package Cinna;
    our $bar;	# Declares $bar in current package
    $bar = 'HgS';	# ok, global declared via pragma

The local() generated a compile-time error because you just touched a global name without fully qualifying it.

Because of their special use by sort(), the variables $a and $b are exempted from this check.