The default access mode when you pass only a string to open is reading. From perldoc -f open:

If MODE is `'<'' or nothing, the file is opened for input. If
MODE is `'>'', the file is truncated and opened for output,
being created if necessary. If MODE is `'>>'', the file is
opened for appending, again being created if necessary. You can
put a `'+'' in front of the `'>'' or `'<'' to indicate that you
want both read and write access to the file; thus `'+<'' is
almost always preferred for read/write updates--the `'+>'' mode
would clobber the file first. You can't usually use either
read-write mode for updating textfiles, since they have variable
length records. See the -i switch in the perlrun manpage for a
better approach. The file is created with permissions of `0666'
modified by the process' `umask' value.
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