if ( $_ =~ /^NEW YORK:\s+(\S+)/ ) {
            $newyork = $1;
            $newyork = $newyork || 'NA';
            next;
    }
[download]

(\S+) will always match at least one character and unless you have a team named '0' (the digit zero) then 'NA' will never be assigned.

How about like this UNTESTED:

#!/usr/bin/perl
use warnings;
use strict;

my %lookup = (
    'NEW YORK' => 1,
    'MIAMI'    => 1,
    );

my %data;
while ( <DATA> ) {
    next unless /^([^:]+):\s+(\S+)/ && $lookup{ $1 };
    $data{ $1 } = $2;
}

__END__
NEW YORK: knicks
CHICAGO:        bulls
MIAMI:   heat
LA:        lakers
[download]