Precedence. 'or' has a higher precedence as '=', but '||' has a hihger precedence. Both operators force scalar context, but:

 @foo = @bar or warn "Oeps";

can be read as

 (@foo = @bar ) or warn "Oeps";,

assigning the contents of @bar to @foo and then evaluating the array @foo in scalar context, resulting in the length of @foo.

On the other hand,

 @foo = @bar || warn "Oeps";

will be evaluated as:

 @foo = (@bar || warn "Oeps";

assigning the length of @bar (if it has any!) to the first element of @foo.

HTH Paul