Point of order: It doesn't return null (undef in Perl terminology), it returns a special value which evaluates as an empty string when a string is expected (such as when printing) or as the number 0 when a number is expected¹.

If you want to print "0" instead of an empty string, you need to force it to be used as a number first:

my $overwrite = (length($function)>1) + 0;
[download]

or use the ternary operator (which also allows you to set values other than 1/0, if you want to):

my $overwrite = length($function)>1 ? 1 : 0;
[download]

Note, though, that this is only useful for making printed output look the way you expect. The original value returned by > will work perfectly well for flow control purposes (e.g., in an if condition) without needing to be converted to a plain 0 first.

 

¹ Yes, a normal empty string is also evaluated as 0 when a number is expected, but, if warnings are active, you'll get an "argument isn't numeric" warning when doing so. The "special" value returned by false boolean operators does not generate this warning.