17:35 >perl 678_SoPW.pl

With parentheses:

@x = (Initial list of words 1)
$y = 'Initial string 1'

foo called in scalar context
@z = (String_from_foo)

After assignment to LHS:

@x = (Words_z from conditional)
$y = 'Initial string 1'

Without parentheses:

@x = (Initial list of words 2)
$y = 'Initial string 2'

foo called in scalar context
@w = (String_from_foo)

After assignment to LHS:

@x = (Words_w from conditional)
$y = 'Initial string 2'

17:35 >perl -v

This is perl 5, version 18, subversion 0 (v5.18.0) built for MSWin32-x86-multi-thread-64int