Backslash characters are not what you think they are in double-quoted strings. You probably just want to open using Unix/Linux style, which is recognized by windows, or you should use File::Spec.

open DIR, "/mail";
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or maybe even use glob (). I have no idea how well that works on Windows

foreach my $f (glob ("/mail/*.*")) {
    open my $fh, "<", $f;
    print scalar <$fh>;
    }
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