There's a bit that gets set when you 'import' a variable and this bit tells 'strict' that the variable has been 'declared'. use vars simply 'imports' the variable.

The heuristic that perl uses to detect when a variable is being 'imported' is to detect when the variable slot of a glob gets assigned to by code compiled into another package. Note that the importing has to be done before the code that uses the imported variable is compiled.

 > perl -w
use strict;
BEGIN { *x= \$foo::y; }
print $x;
Variable "$x" is not imported at - line 3.
__END__
Global symbol "$x" requires explicit package name at - line 3.
Execution of - aborted due to compilation errors.

 > perl -w
use strict;
BEGIN { package foo; *main::x= \$foo::y; }
print $x;
__END__
Name "foo::y" used only once: possible typo at - line 2.
Use of uninitialized value in print at - line 3.
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