The real issue you have is that when the hash is empty, @keyarr gets no values from keys %hash. Thus, $keyarr[0] is undefined, causing the warning.

However, when the hash is not empty, @keyarr has values, so $keyarr[0] is defined and doesn't trigger the warning.

Add a print statement to show what's in @keyarr and then it may be clearer to you what is happening.

my (%hash);
#my (%hash) = ( foo => 'bar' ); # Try this variation
my (@keyarr)=(keys (%hash));

print "keys: '@keyarr'\n";      # Add this line

if (defined ($hash{$keyarr[0]}))
{print "hash is defined\n";}

else
{print "hash is undefined\n";}
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