Just to pick a nit:

Localizing is pointless when it happens at top level

Not entirely, e.g. if you've got a file foo.pl that does local $/, and a file bar.pl that does do "foo.pl" (any of eval/do/require/use), then bar.pl is protected from the change to $/.

I know you also said "a program that consists in just 1 file", but I'd say foo.pl doesn't know if it's being run by perl foo.pl or by being done from another script, or perhaps being run in a persistent process. So localizing would be playing it safe.