No they cannot be used interchangeably in all places, the word versions are specifically guaranteed to have lower precedence than the symbol versions (infact guaranteed to have the lowest precedence of any operators), you might care to try for instance:
open ONE, "somefilethatdoesnexist" or print "one: $!\n";
open TWO, "somefilethatdoesnexist" || print "two: $!\n";
As you can see the latter doesn't behave as expected as the '||' binds more tightly and ends up taking the value of the filename - infact the check is optimised away because this is essentially a no-op, as the output from B::Deparse shows:
print "one: $!\n" unless open ONE, 'somefilethatdoesnexist';
open TWO, 'somefilethatdoesnexist';
Jeffa's response shows the difference -- because of the way those ops are weighted for ordering, depending on what statements you use them in, they end up meaning different things. In the source that is shown as on the OP's example there is no difference in execution -- but that can't be said in a blanket statement.
For a very thorough explaination of just what the differences are between the two (just an issue with precedence, which has already been covered), and the order of precedence for all perl operators, see the perldoc perlop
kyoshu, please don't delete your posts this way. It makes the following answers look out-of-context, and deny the possibility to learn to the other readers. Many times one learns from errors, even others' ones.
It's better to put a pen strike over the sentences you don't agree any more like this, which can be obtained via the strike tag: