fix string comparisons with $] to use numeric comparison instead

[book]

The fix follows Zefram's suggestion from
https://www.nntp.perl.org/group/perl.perl5.porters/2012/05/msg186846.html

On older perls, however, $] had a numeric value that was built up using
floating-point arithmetic, such as 5+0.006+0.000002. This would not
necessarily match the conversion of the complete value from string form
[perl #72210]. You can work around that by explicitly stringifying
$] (which produces a correct string) and having that numify (to a
correctly-converted floating point value) for comparison. I cultivate
the habit of always stringifying $] to work around this, regardless of
the threshold where the bug was fixed. So I'd write

use if "$]" >= 5.014, warnings => "non_unicode";