use strict;
use warnings;
my $z;    # $z contains undef
if ($z eq "") {    # oops, Use of uninitialized value $z in string eq
    print "1. string is empty (with warnings)\n";
}
# Switch off the specific uninitialized warning in a lexical scope.
{
    no warnings 'uninitialized';
    if ($z eq "") {    # no warning this time
        print "2. string is empty (with no warnings)\n";
    }
}
# warnings switched back on at end of scope