These two do the same thing:

my $x;
my ( $x );
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The second one is easier to add more variables to later. These are the same:

my $x ; my $y;
my ( $x, $y );
# NOT: my $x, $y
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These two are a little different:

my   $x   = foo();
my ( $x ) = foo();
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The difference between them is that the second puts foo() in a list context while the first puts foo() in a scalar context.

In this particular case (my ($x) = $y =~ /(p)/), the list context is important because that's how the captures come back. In a scalar context, you'd just get a boolean value to tell you whether the pattern matched.