#!/web/soft/bin/perl

%hash = ( 1 => 5,
          2 => 10,
          3 => 15,
          4 => 20 );

if( defined( $hash{4} ) ) {
    print "defined 1\n";         # prints
}

undef( $hash{4} );

if( defined( $hash{4} ) ) {
    print "defined 2\n";         # does not print
}