It doesn't seem to be the same as $|.

How did you arrive at that conclusion?

Given that the vars in perlvar are package variables found in every package, they should be.

>perl -le"for (0,1,0) { $|=$_; print $|, ${'main::|'} }"
00
11
00
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>perl -le"print \$| == \${'main::|'} ? 'same var' : 'diff var'"
same var
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Update: Added second snippet.

How did you arrive at that conclusion?

I had simply tried

$ perl -e '$|=1; print "$main::|\n";'
|
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On second thought, it's obviously a parsing issue, i.e. $main::| is not being parsed as ${'main::|'}, but rather as $main::, so the '|' is being printed literally.

You can see form the trailing "|" that you were referencing $main:: and not $main::|, so there's no ground for drawing conclusions about relation between $main::| and $|.