Actually, there's a fairly explicit explanation in `perldoc perlnumber`:

`Arithmetic operators
The binary operators "+" "-" "*" "/" "%" "==" "!=" ">" "<" ">=" "<
+="
and the unary operators "-" "abs" and "--" will attempt to convert
arguments to integers. If both conversions are possible without lo
+ss
of precision, and the operation can be performed without loss of
precision then the integer result is used. Otherwise arguments are
converted to floating point format and the floating point result i
+s
used. The caching of conversions (as described above) means that t
+he
integer conversion does not throw away fractional parts on floatin
+g
point numbers.
++ "++" behaves as the other operators above, except that if it is a
string matching the format "/^[a-zA-Z]*[0-9]*\z/" the string
increment described in perlop is used.
Arithmetic operators during "use integer"
In scopes where "use integer;" is in force, nearly all the operato
+rs
listed above will force their argument(s) into integer format, and
return an integer result. The exceptions, "abs", "++" and "--", do
not change their behavior with "use integer;"
`

Note the exception for "++" -- which several of us, esp. /me -- forgot to mention...and the additional exceptions while using `use integer`.

BTW, there's some related matter (much less clear, to me, anyway) in `perlvar`... and perhaps (probably?) in other docs? How say you, Monks?

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