Recently, jdporter closed one of his comments with the following snippet: $orry - not $orry

This is a valid Perl expression. So, I wondered, what is its value? My first thought was "easy: that's just $orry converted to a number", but as ever so often, my first thought is wrong. If $orry = 0, then the value is -1. If $orry is a string, then the value is 0. Unless the string is empty, in which case the value is -1. And unless the string starts with a number, where the result is that number, even if the number is 0. And then, there are references, typeglobs, tied scalars, and objects, which may even overload stuff.

Here's a collection of expressions for $orry, and results of $orry - not $orry.

                           $orry : $orry - not $orry
                              42 : 42
                              -1 : -1
                               0 : -1
                           undef : -1
                              "" : -1
                             0e0 : -1
                           "0e0" : 0
                          "a123" : 0
                        "-.2e2e" : -20
                     "nancy boy" : NaN
                    "infidelity" : Inf
                         *STDOUT : 0
                              [] : 94269931603432
                 File::Temp->new : 94269935380840
  URI->new(q(https://perl.org/)) : 0

[download]

The implicit conversions of scalars into numeric / boolean / string context are amazing, and sometimes scary. Fortunately, use warnings; detects most of the surprising ones.

Update:: syphilis contributed the strings "nancy boy" and "infidelity" which are indeed amusing additions. I inserted them after the strings starting with numbers. In the case of 'nancy boy' my text "string starts with a number" looks silly because NaN claims to be not a number.

In reply to Fun with implicit type conversions by haj

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