use Tie::Hash; 
tie my %h, 'Tie::StdHash'; 

# Check with an empty %h first...

print "TRUE 1\n" if ( %h );    # Nope not true.
print "TRUE 2\n" if ( ()=%h ); # Not true either.

# Now a non-empty %h ...

$h{foo} = "bar";
print "TRUE 3\n" if ( %h );    # Nope not true.
print "TRUE 4\n" if ( ()=%h ); # Yes!