It is not guaranteed. From 'perldoc -f sysopen' on 5.005_03 I get: 
=item sysopen FILEHANDLE,FILENAME,MODE

=item sysopen FILEHANDLE,FILENAME,MODE,PERMS

[ snip ]

The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module C<Fcntl>.
For historical reasons, some values work on almost every system
supported by perl: zero means read-only, one means write-only, and two
means read/write.  We know that these values do I<not> work under
OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
use them in new code.

[download]
So those values probably work. But not always.

In reply to Re (tilly) 3: Two-arg open() considered dangerous by tilly
in thread Two-arg open() considered dangerous by chip

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