There are two good reasons. The first is backwards compatibility. Perl tries very hard not to break old programs, and there are a lot of old programs that would be broken by such a change.

The second reason is that as it is now, a program as simple as 

 
while(<>) {
    print;
}

[download]

Just works, ie it print out the same data as it reads. If STDOUT defaulted to UTF-8, it would also need to default to UTF-8 for reading operations.

And when that's the default, suddenly reading a non-UTF-8 file will cause either a fatal error, or that the data can't be interpreted correctly.

Perl 6 - second systems done right

In reply to Re: why no default unicode? by moritz
in thread why no default unicode? by perl-diddler

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