in reply to my $x or my ($x)

The variable itself isn't in a list context; whatever comes after the equal sign is in a list context. Regular expression matches are one place where this matters, but here's a clearer example:
$ perl -le 'my $x = localtime; print $x' Tue Apr 4 11:25:49 2006 $ perl -le 'my($x) = localtime; print $x' 52

If you read the docs for localtime, you'll see that it returns different values when called in a scalar or list context, so the type of assignment can make a big difference.

Here's another example:

#!/usr/bin/perl sub test { if (wantarray) { return ('list context'); } else { return 'scalar context'; } } my $s = test; print "s=$s\n"; my(@l) = test; print "l=@l\n";
which outputs:
s=scalar context
l=list context