my $counter = 0; # declare and initialize lexical variable
use vars qw($counter); # declare global variable living in symbol table
                                                                                                                                              
$counter = 0; # initialize again, just to show that $counter in 'use vars' is not affected

print "\$main::counter: $main::counter\n"; # $counter living in symbol table: not initialized
$main::counter = 1; # initialize $counter in symbol table
print "\$main::counter: $main::counter\n"; # prints 1
print "\$counter: $counter\n"; # prints 0